Infinite creature LTBInfinite creature ETBInfinite creature sacrifice triggersInfinite death triggersInfinite copies of creature tokens you controlInfinite power for certain creatures until end of turn
How it works
Activate Xavier Sal's first ability by tapping it and removing a +1/+1 counter from Hyrax Tower Scout if it has one, or any counter from another permanent you control otherwise.
Xavier Sal triggers, causing it and all other creatures you control that share a creature type with it to get +2/+0 and gain undying until end of turn.
Resolve the Xavier Sal ability, causing you to populate.
Activate Yahenni by sacrificing Hyrax Tower Scout.
When Hyrax Tower Scout dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
When Hyrax Tower Scout enters the battlefield, it triggers, untapping Xavier Sal.