Demon's Horn

• WWizards of the Coast2009-10-01
  Official Ruling
  The ability triggers whenever any player, not just you, casts a black spell.
• WWizards of the Coast2009-10-01
  Official Ruling
  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.

Rulings via Scryfall