Xathrid Necromancer + Ashnod's Altar + Maskwood Nexus

Xathrid Necromancer

Ashnod's Altar

Maskwood Nexus

How it works

1. Activate Ashnod's Altar by sacrificing any other creature.
2. When the creature dies as a Human due to Maskwood Nexus, Xathrid Necromancer triggers, creating a 2/2 Zombie creature token.
3. Repeat.

This loop turns any sacrifice outlet into a self-sustaining engine by completely rewriting the creature types of your entire board using Maskwood Nexus. Because Maskwood Nexus makes every creature you control a Human, Xathrid Necromancer treats every single sacrifice as the death of a Human, immediately replacing the lost soul with a fresh Zombie token. You can then feed that Zombie right back into Ashnod's Altar to generate colorless mana while keeping the loop going indefinitely.