Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Whenever a source deals damage to Reaper of Sheoldred, that source's controller gets a poison counter.

Menace

When Sheoldred enters the battlefield, each opponent sacrifices a nontoken creature or planeswalker.

: Exile Sheoldred, then return it to the battlefield transformed under its owner's control. Activate only as a sorcery and only if an opponent has eight or more cards in their graveyard.

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)

At the beginning of your upkeep, return target creature card from your graveyard to the battlefield.

At the beginning of each opponent's upkeep, that player sacrifices a creature.

Menace

Toxic 2 (Players dealt combat damage by this creature also get two poison counters.)