
You gain 1 life. Target creature gets -X/-X until end of turn, where X is the amount of life you gained this turn.
That feeling of something on your chest is usually sleep paralysis. Occasionally, it's something else.
Illustrated by Igor Kieryluk
Released on July 13, 2018
Nightmare’s Thirst counts how much life you’ve gained this turn, not how much your life total changed. For example, if you’ve gained 5 life and lost 8 before Nightmare’s Thirst resolves, Nightmare’s Thirst has you gain 1 more life, then it gives the target creature -6/-6.
If the target creature is an illegal target when Nightmare’s Thirst tries to resolve, the spell doesn’t resolve. You won’t gain 1 life.