Tainted Remedy

If an opponent would gain life, that player loses that much life instead.

Rite of Consumption

As an additional cost to cast this spell, sacrifice a creature.
Rite of Consumption deals damage equal to the sacrificed creature's power to target player or planeswalker. You gain life equal to the damage dealt this way.

Plains

Wall of Shards

Defender, flying
Cumulative upkeep—An opponent gains 1 life. (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)

Life Burst

Target player gains 4 life, then gains 4 life for each card named Life Burst in each graveyard.

Dark Ritual

Swamp

Congregate

Target player gains 2 life for each creature on the battlefield.

False Cure

Until end of turn, whenever a player gains life, that player loses 2 life for each 1 life they gained.

Vizkopa Guildmage

{1}{W}{B}: Target creature gains lifelink until end of turn.
{1}{W}{B}: Whenever you gain life this turn, each opponent loses that much life.

Laquatus's Champion

When Laquatus's Champion enters, target player loses 6 life.
When Laquatus's Champion leaves the battlefield, that player gains 6 life.
{B}: Regenerate Laquatus's Champion.