Cheapest priced copy (Darksteel): $9.49 at Card Kingdom, near mint, their price list as of .
Nemesis Mask 
Artifact — Equipment · Uncommon · Darksteel · #134
All creatures able to block equipped creature do so.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper