Thieving Amalgam

At the beginning of each opponent's upkeep, you manifest the top card of that player's library. (Put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)
Whenever a creature you control but don't own dies, its owner loses 2 life and you gain 2 life.

Umbilicus

At the beginning of each player's upkeep, that player may pay 2 life. If they don't, they return a permanent they control to its owner's hand.

Plague Reaver

At the beginning of your end step, sacrifice each other creature you control.
Discard two cards, Sacrifice Plague Reaver: Choose target opponent. Return Plague Reaver to the battlefield under that player's control at the beginning of their next upkeep.

Ruthless Winnower

At the beginning of each player's upkeep, that player sacrifices a non-Elf creature of their choice.

The Abyss

At the beginning of each player's upkeep, destroy target nonartifact creature that player controls of their choice. It can't be regenerated.

Woebringer Demon

Flying
At the beginning of each player's upkeep, that player sacrifices a creature of their choice. If the player can't, sacrifice Woebringer Demon.

Smokestack

At the beginning of your upkeep, you may put a soot counter on Smokestack.
At the beginning of each player's upkeep, that player sacrifices a permanent of their choice for each soot counter on Smokestack.