Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight | Umbral Collar Zealot

Cards

All permanents on the battlefield (
Eirdu as Isilu, Carrier of Twilight and
Strangleroot Geist without a +1/+1 counter on it).

Activate
Umbral Collar Zealot by sacrificing
Strangleroot Geist.
When
Strangleroot Geist dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Umbral Collar Zealot ability.
Activate
Umbral Collar Zealot by sacrificing
Strangleroot Geist.
When
Strangleroot Geist dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Strangleroot Geist enters and dies as a state-based action due to having 0 toughness. Its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Umbral Collar Zealot ability from step 4.
Repeat from step 4.

If
Strangleroot Geist starts with a +1/+1 counter on it, then you may start the combo from step 4 instead.

This combo has been estimated to be Exhibition (Bracket 1+).