Its curve mimics the twists of life and death.

Demon's Horn

Whenever a player casts a black spell, you may gain 1 life.

Duel Decks Anthology: Divine vs. Demonic

Magic 2012

Salvat 2011

Magic 2011

Duels of the Planeswalkers

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.

The ability triggers whenever any player, not just you, casts a black spell.