Essence Symbiote

Whenever a creature you control mutates, put a +1/+1 counter on that creature and you gain 2 life.

Ashnod's Altar

Nethroi, Apex of Death

Mutate {4}{G/W}{B}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Deathtouch, lifelink
Whenever this creature mutates, return any number of target creature cards with total power 10 or less from your graveyard to the battlefield.

Mysterious Egg

Whenever this creature mutates, put a +1/+1 counter on it.

Altar of Dementia

Sacrifice a creature: Target player mills cards equal to the sacrificed creature's power.