If Invulnerability were really really desired in a campaign, I would recommend using the 'Create New Power' rules in FREd.
Figure out how much it costs to be Invulnerable to an SFX using Desolid vs Damage Type, and then redefine it as "Invulnerability" with a behavior tailored to the effect desired.
So, Desolid, Only to Protect against Limited Form of Attack, 0 End, Persistant gets us close. Then, we will want to "Buy off" the built in "cant affect Physical World" limitation of Desolid; the book indicates that a GM can insist on making people taking Desolid to mimic Invulnerability buy Affects Physical World on everything else or not as they see fit. Im betting most GMs dont, however Ill comprimise with a +1 Advantage to accomplish this half way between buying it off and not buying it off. Usually with Desolid you have to define a SFX that will affect the character, but we can skip that with our version because this form of Desolid is explicit rather than implicit; in other words ALL other SFX hurt the character normally EXCEPT the one the Invulnerability is bought against.
<table border="0" cellpadding="0"><tr><td align="right"><b>Cost  </b></td><td><b>Power</b></td><td><b>END</b></td></tr><tr><td align="right" valign="top">60  </td><td><b><i>Invulnerability: </i></b>Desolidification , Reduced Endurance (0 END; +1/2), Persistent (+1/2), Buy off Cant affect Physical World (+1) (120 Active Points); Only To Protect Against Limited Type Of Attack (-1) </td></tr></table>
<b>Power Cost:</b> 60
OK, so we define Invulnerability:
<table border="0" cellpadding="0"><tr><td align="right" valign="top">60  </td><td><b><i>Invulnerability: </i></b>Custom Power (60 Active Points) Constant, Self Only, 0 END Persistent [<b>Notes:</b> A character with this Power is immune to the direct effects of any Power directed against him with a Specific tightly defined SFX, whether that effect would be beneficial or not. This includes any of the characters own powers which are not Self Only or Sensory in nature; thus Instant powers such as Aid and Healing cannot be used on oneself unless specifically limited to Self Only if they are of a SFX covered by the Invulnerability.] </td><td valign="top" align="right"></td></tr></table>
This compares well to 75% Resitant Dmg Reduction for PD or ED, which is more generally broadbased, but doesnt stop all damage, and Invulnerability will protect vs some powers that dont fall under Dmg Reduction because they dont deal in pure damage.
A character with this power would be Invulnerable to a single tight SFX. Thus 'Mutant powers' may or may not be tight enough depending on how prevalent mutants are in a campaign setting; similarly 'Magic' is rather broad and will not be tight enough in a Magic-heavy campaign but would be fair in a campaign where Magic was rarer. See below for more on this. For example, the Marvel hero Collossus is Invulnerable to Magic (for all intents and purposes); Magic is fairly uncommon in the Marvel universe (it comes up, but is relatively Uncommon) so this is acceptibly tight in that setting.
An Invulnerable Character still takes Knockback normally and Invulnerability does not provide protection from Knockback damage unless the SFX of the Invulnerability covers Impact damage or Knockback damage (Invulnerability to Knockback Damage, 60 Active Points, Nonpersistant (-1/2), Activation 14- (-1/2), 30 real cost). Characters that want to represent a complete nullification of effects by a SFX to include not being affected by Knockback should buy Knockback Resistance Linked to thier Invulnerability.
A few Modifiers might be specifically appropriate to Invulnerable.
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Scope: The broadness of the SFX defined should have an effect on the cost of the power. A reasonably tight single SFX should represent a SFX that approximately 10-15% of the potential opposition possess within the campaign as whole. Thus, SFX: Heat/Fire, SFX: Radiation, SFX: Magnetism are all tight single SFX in most supers campaigns but SFX: Mental or SFX: Mutant Powers are usually not as they cover a broader range of powers/abilites unless mutants or mental powers are abnormally scarce. Similarly Invulnerability to "My Brothers Mutant Powers" is much more specific and should be less expensive. To affect a broader or more narrow SFX use the following chart:
<table border=0 cellpadding=0><tr><th>Scope</th><th>Modifier</th></tr><tr><td>Vs. a tight SFX and a single specific Individual with that SFX</td><td>-2</td></tr><tr><td>Vs. a tight SFX and a Small Group (family, fellow test subjects) with that SFX</td><td>-1 1/2</td></tr><tr><td>Vs. a broad SFX and a single specific Individual with that SFX</td><td>-1 1/2</td></tr><tr><td>Vs. a broad SFX and a Small Group (family, fellow test subjects) with that SFX</td><td>-1</td></tr><tr><td>Vs. a tight SFX and a Group (race of people or a large sub-race) with that SFX</td><td>-1</td></tr><tr><td>Vs. a broad SFX and a Group (race of people or a large sub-race) with that SFX</td><td>-1/2</td></tr><tr><td>broad SFX (appoximately 25% prevalent)</td><td>+1/2</td></tr><tr><td>very broad SFX (appoximately 50% prevalent)</td><td>+1</td></tr><tr><td>extremely broad SFX (appoximately 75% prevalent)</td><td>+1 1/2</td></tr><tr><td>encompassingly broad SFX (appoximately 100% prevalent)</td><td>+2</td></tr></table>
For example, SFX:"Blunt" or "Impact" (however you want to phrase it) would be a SFX that stops, among other things (including Knockback and falling damage), STR based damage; practically EVERYTHING can do this type of damage in HEROs, so this is an Encompassingly Broad SFX (+2) and costs 180 points. {Off hand, that seems fair to me for the magnitude of such a power.}
However, SFX:"Slashing/Piercing" or "Edged" (again, semantic of choice) damage is a tighter SFX; maybe 50% to 75% of the opposition will have a piercing/slashing weapon (including projectile weapons). In most supers, with an emphasis on non-lethal I would go with 50%, but in a Fantasy or Dark Champs setting 75%. Thus this would cost 120 to 150 points depending on setting.
Thus, in a normal supers campaign with more non-lethal attacks, it would cost 300 points to be Invulnerable to Blunt and Edged Damage which is pretty encompassing. Points wise it is equivalent to 100 PD Hardened x4 Armor.
SFX:"All Energy" is similar to "Edged", but reversed. In most supers campaigns many enemies will have Energy attacks, whereas in Fantasy or Dark Champs, etc they will be less prevalent; therefore 75% in supers, 50% in more realistic settings where EBs and RKAs vs ED are less common. Thus this Invulnerability would cost 150 points in a Champs game.
To be Mr. Invulnerable with no offsetting Limitations would therefore cost 450 points in most Champsion games, granting across the board protection from Physical and Energy attacks regardless of source. If that character applied even a single Limitation such as Visible to all 3 forms of Invulnerable detailed above, it would cost 360 points, and when combined with Nonpersistent (-1/2) comes in at 257 points, leaving almost 100 points for Strength Linked to Invulnerable and sundry other abilities in a 350 point starting game.
SFX: All Superhuman Powers (ie anything bought as a non-FOCI/item based superhuman ability built using the Powers rules) is obviously a +2 Scope. However, it is important to note that Invulnerability only protects against affects that apply effect or damage directly, and not the secondary benefits of such powers. Thus an opponent with Density Increase or Strength bought as a Power has great strength, and when striking with strength damage would still affect a character with this type of Invulnerability. So for example, Collossus's strength-based strike, Wolverine's Claws (bought as SFX: metal claws, not SFX: mutant power), and Cyclops shooting a tree branch overhead to fall upon such a character would all do damage as normal, but all direct Power Attacks such as Storm's lightning Bolts, Psylockes Psi-blade, or Leech's Power Suppress would fail to affect a character with this Encompassing SFX Invulnerability. As a secondary example, someone with Invulnerability to all Mutant Powers (depending on campaign, ususally either Very Broad, 120 pts or Broad, 90 pts) in the same situation would also be immune to Storm, Psylocke, and Leech since all of those characters are using Mutant Powers, but would not be safe from the Human Torch's Nova Blast because that does not have a Mutant Power SFX; the character with the Invulnerability to all Superhuman Powers would be completely safe from it.
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Hulled: If a Character or Vehicle's Invulnerability stems from a protective outer skin or outer layer and thier insides do not benefit from any sort of Invulnerability, having only any normal defences, then that is roughly equivalent to a mild form of Restrainable and is worth a -1/4 Limitation. Any attack, including some Conforming AoEs (such as gas clouds and similar), some Change Environments, and some Indirects which can circumvent the Invulnerable Hull are applied normally, ignoring the Invulnerability. Further, some circumstances may circumvent the Invulnerability for all attacks outright, for example an Invulnerable Hulled Vehicle with an open door would lose the benefit of its Invulnerability to an attack that was able to exploit such an opening, typically with a -2 to hit or with a Called Shot if using Hit Locations. Finally, opponents that have successfully Grabbed an Invulnerable character or Vehicle may strike at openings in the Hull with the usual penalties for Grabbing while keeping a hand free if they can reasonably reach those openings. For most characters, as opposed to Vehicles, openings in thier 'Hull' would include thier eyes, nostrils, ears, mouth, and other bodily orifices at the GMs discretion.
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Weakness Some Invulnerable Characters have a specific exclusion clause to thier Invulnerability. For some, it is a literal location on thier body (an Achille's Heel), for others its a specific material (such as Baldur and Mistletoe or Superman and Kryptonite), and for still others its more abstracted. If a character has a single location on thier body that is not protected by thier Invulnerability, this can be best represented by using a process similar to the Focus rules. The spot may be either Obvious or Inobvious, and either Accessible or Inaccessible. It is usually appropriate to also take either a Vulnerability or Susceptibility (or both) revolving around the Weakness to depict a truly serious frailty.
An Inobvious Inaccessible Weakness is a -0 Limitation; it represents a Weakness which is neither apparent or exploitable in combat and is not limiting enough to gain points from. Such a Weakness basically serves as a flavorful character hook or as an enabler for a larger concept; for example a character with such a Weakness may take a Physical Limitation that occurs if the Weakness is exploited as Infrequent or something similar.
An Obvious Inaccessible Weakness is worth -1/4; it represents a part of the characters body that is noticably unprotected or
vulnerable, such as thier eyes perhaps, but which is generally difficult or impossible to target in combat; suprise attacks or attacking the character while they are incapacitated will do the job; this form of Weakness can be used to represent a character that must concentrate to maintain thier Invulnerability, like Captain Britain for example, and when caught by suprise or incapacitated are as vulnerable as anyone else (aside from any other defenses they may have).
An Inobvious Accessible weakness is worth -1/2; it represents a Weakness which is not obvious or apparent but which can be targeted in combat to circumvent the Invulnerability. There are two basic types of Inobvious Accessible Weaknesses, Locations and Materials. Location Weaknesses are a place on the body of a character which is not protected by the Invulnerability; an opponent that knows about the locations may take a -2 OCV on attack rolls if not using Hit Locations, or may make a called shot if using Hit Locations to target this area and if successful will completely bypass the characters Invulnerability. Material Weaknesses represent an Invulnerability which does not affect a specific type of material such as alluminum, silver, or wood; a weapon constructed of that material (even a makeshift weapon) will always circumvent the Invulnerability. Superman's weakness to Kryptonite or the Norse god Baldur's weakness to mistletoe are examples of this type of Weakness (Material), as is the classic Achille's Heel (location).
An Obvious Accessible Weakness is worth -1; it represents a weakness which is both clearly identifiable and exploitable in combat. By taking a -2 OCV to thier attack rolls or by making a called shot if using Hit Locations, opponents can circumvent the characters Invulnerability entirely. Such a weakness is rare, but examples include the classic Power Suit\Super Body Armor without a Helmet or face mask.
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Some existing Modifiers might behave in interesting ways with this power. Some of them are listed below:
Affects Desolidified: Though derived from Desolid, Invulnerability is now a seperate power and is not affected by powers with this Advantage.
Area of Effect: Normally this has no bearing on Invulnerability, but Areas of Effect that are bought Conforming do have an effect on Hulled Invulnerability. A Conforming attack treats a Hull as a wall, but may at the GMs discretion seep into any breaches or openings in a Hull circumventing the Invulnerability and having its normal (or a reduced) effect. For a character with a Hulled Invulnerability, this might indicate a poison gas or a fire ball which gets into his respiratory system even though his skin is unharmed.
Armor Piercing: AP has no effect vs a character Invulnerable to the SFX of the AP attack.
AVLD: AVLD has no effect vs a character Invulnerable to the SFX of the AVLD attack. Invulnerable is not a 'Defense' per se and thus cannot be defined as the targeted Defense of an AVLD attack.
BOECV: If the Invulnerability applies to the SFX of the BOECV attack, then the BOECV attack has not effect.
Inherent: Invulnerability can be bought Inherent.
Indirect: Normally Inderect is immaterial to Invulnerability, but can interact with Invulnerability if the Hulled Limitation is taken with Invulnerability. If an Indirect Attack is bought in such a way to circumvent a targets 'outer layer', or which indicate a particularly accurate form of attack which is 'guided' then as circumstances indicate Indirect of this sort will allow the circumvention of Hulled Invulnerability. Often the Invulnerable character can take steps to minimize this flaw by closing any breaches in thier Hull but at the GMs discretion a suitable Indirect attack may continue to exploit this weakness by taking an appropriate penalty to thier attack roll ranging from -2 OCV up even if the Invulnerable character is attempting to prevent it.
NND: NND has no effect vs a character Invulnerable to the SFX of the NND attack. Invulnerable is not a 'Normal Defense' per se and thus is not circumvented by an NND attack if the SFX of the NND Attack is covered by the SFX of the Invulnerability.
Penetrating: Penetrating has no effect vs a character Invulnerable to the SFX of the Penetrating attack.
Personal Immunity: This Advantage may be taken on Invulnerability, although it is a limited use case. This would allow a character to affect themselves with a targeted beneficial effect if the SFX of the Invulnerability would normally prevent it. The primary use for this would be if a character had an Aid or Healing Power not bought Self Only which had a SFX covered by thier own Invulnerability.
Usable On Others: This can be applied normally to Invulnerability. However, some duration or other means of ending the effect should be defined as normal for a 0 END Constant Power.
Variable Special Effects: At the GMs discretion, this Advantage may be allowed to represent an Invulnerability which may be 'fine tuned'. However, this could be very powerful and should be considered a STOP sign ability.
Ablative: This is not appropriate to Invulnerability; a better way to represent an "Ablative" Invulnerability is using either a Burnout Activation or Charges.
Activation: Activation Rolls are appropriate to Invulnerability, particularly with either the Jammed or Burnout rules. Burnout particularly could be used to model an "Ablative" coating.
Charges: In general, since Invulnerable is already a 0 END Constant power, Charges are inefficient. However, they can be taken with Invulnerable if desired, and is another method that can be used to represent an 'Ablative' coating or even a '9 Lives' effect. However, the Fuel option should not be allowed, or at least treated as a Yield sign power. Boostable Charges have no relevance to Invulnerability.
Nonpersistant: This can be taken with Invulnerability, and can also be combined with Concentration, Costs Endurance only to ACtivate, Increased Endurance, and Extra Time to represent a kind of Invulnerability that takes some effort to 'turn on', and when combined with Visible may indicate some kind of Armorskin or a metamorphosis into some more durable form.
FOCI: Foci works normally with Invulnerability unless the Weakness modifier is also taken on Invulnerability. A GM should carely examine an Invulnerability with both FOCI and Weakness to ensure that they are 2 seperate issues and not a 'double dip'. Typically a FOCI will benefit from whatever Invulnerabilities it grants. Thus an Amulet of Fire Prevention which grants Invulnerability to Fire would itself be immune to fire damage. This is open to GM's interpretation however, particularly in the case of a Fragile FOCI.
Gradual Effect: This is not appropriate for Invulnerability.
OIHID: This Limitation is appropriate to Invulnerable, but see Visible below and Nonpersistent above.
Restrainable: This is not normally appropriate to Invulnerability; see Weakness instead.
Visible: Much like Armor, Visible Invulnerability takes the form of some outward indication that the target is particularly resistant to harm. Unlike Armor, the visual appearance should give a very strong clue of the appropriate types of damage the Invulnerable character is resistant to. Thus someone with Visible Invulnerability to fire may have red skin and constantly flaming hair as a SFX. Someone with a Visible Invulnerable to Mental may have an effect apparent to any with Mental Awareness such as a specific aura or a constant mental static surrounding them. The net effect is anyone possessing a power of the SFX that the Invulnerable character is protected from should be able to 'guess' thier Invulnerability by making an Intelligence check, PER Roll, Power Skill check, or even automatically as the GM decides before deciding to attack them.