Haunted One

Commander creatures you own have "Whenever this creature becomes tapped, it and other creatures you control that share a creature type with it each get +2/+0 and gain undying until end of turn." (When a creature with undying dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)

Ellie, Brick Master

Distract the Horde — Whenever a player attacks one of your opponents, that attacking player creates a tapped 1/1 black Fungus Zombie creature token named Cordyceps Infected that's attacking that opponent.
Partner—Survivors (You can have two commanders if both have this ability.)

Joel, Resolute Survivor

Menace
Whenever a creature token dies, put a +1/+1 counter on Joel and draw a card. This ability triggers only once each turn.
Partner—Survivors (You can have two commanders if both have this ability.)

Cabal Ritual

Add {B}{B}{B}.
Threshold — Add {B}{B}{B}{B}{B} instead if seven or more cards are in your graveyard.