Nightmare's Thirst
Instant
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.
Igor Kieryluk
Core Set 2019 (M19)
• #111Rulings
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.
(wotc • 2018-07-13)
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.
(wotc • 2018-07-13)
Rulings
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.
(wotc • 2018-07-13)
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.
(wotc • 2018-07-13)