Mikaeus, the Unhallowed + Glen Elendra Archmage + Yahenni, Undying Partisan

Mikaeus, the Unhallowed

Glen Elendra Archmage

Yahenni, Undying Partisan

How it works

1. Activate Yahenni by sacrificing Glen Elendra Archmage.
2. When Glen Elendra Archmage dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Resolve the Yahenni ability.
4. Activate Yahenni by sacrificing Glen Elendra Archmage.
5. When Glen Elendra Archmage dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. Resolve the Yahenni ability.
7. Repeat.

This loop relies on the fascinating interaction between Glen Elendra Archmage's native persist mechanic and the passive undying ability granted to it by Mikaeus, the Unhallowed. Because +1/+1 counters and -1/-1 counters annihilate each other as a state-based action, the Archmage can alternate between returning with a -1/-1 counter (which is immediately cancelled out by a subsequent undying return) and a +1/+1 counter. Yahenni, Undying Partisan acts as the active sacrifice outlet that triggers this endless cycle, letting you churn out infinite death triggers and sacrifice counts while keeping the Archmage's counter-state completely reset on each iteration.