Mikaeus, the Unhallowed + Soulstinger + Yahenni, Undying Partisan

Mikaeus, the Unhallowed

Soulstinger

Yahenni, Undying Partisan

How it works

1. Activate Yahenni by sacrificing Soulstinger.
2. When Soulstinger dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Soulstinger enters the battlefield, it triggers, putting a -1/-1 counter on it.
4. Resolve the Yahenni ability
5. Repeat.

This clever loop centers around Mikaeus, the Unhallowed, whose static ability grants undying to your non-Human creatures and offsets counter mathematics. When you feed Soulstinger to Yahenni, Undying Partisan's sacrifice ability, Mikaeus brings Soulstinger back with a +1/+1 counter. Soulstinger's own enters-the-battlefield trigger then places two -1/-1 counters on itself, which immediately annihilate both the +1/+1 counter and the previous -1/-1 counter through state-based actions. This leaves Soulstinger completely clean of counters and ready to be sacrificed all over again, generating an endless cascade of deaths, enters-the-battlefield events, and sacrifice triggers while fueling Yahenni with infinite resilience.