Shield of Siberys

(Dragonmarked, page 144, DnD 3.5)

[General]

Your dragonmark protects you against the powers of those cursed with aberrant dragonmarks.

Prerequisite

Any true dragonmark

Benefit

If a creature with an aberrant dragonmark comes within 60 feet of you, you can sense its presence (though not its exact location). In addition, you gain spell resistance equal to 15 + your level in the heir of Siberys prestige class (if any) against the spell-like abilities of aberrant dragonmarks.