Tainted Remedy

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

Plague Drone

Flying
Rot Fly — If an opponent would gain life, that player loses that much life instead.

Double target player's life total. Shuffle Beacon of Immortality into its owner's library.

Vigilance
When Arbiter of Knollridge enters, each player's life total becomes the highest life total among all players.

If a spell or ability would cause its controller to gain life, that player loses that much life instead.

Enchant creature
Enchanted creature gets +3/+3.
Whenever enchanted creature deals combat damage to a player, double its controller's life total.

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

At the beginning of your upkeep, you may have two target players exchange life totals.