Duel Decks Anthology, Divine vs. Demonic — Uncommon
Demon's Horn rulings:
10/1/2009 : The ability triggers whenever any player, not just you, casts a black spell.
10/1/2009 : If a player casts a black spell, Demon’s Horn’s ability triggers and is put on the stack on top of that spell. Demon’s Horn’s ability will resolve (causing you to gain 1 life) before the spell does.