Hellkite Charger + Bear Umbra

Hellkite Charger

Bear Umbra

How it works

1. Declare Hellkite Charger as attacker.
2. Bear Umbra and Hellkite Charger triggers.
3. Resolve the Bear Umbra's trigger, untapping all lands you control.
4. Resolve the Hellkite Charger's trigger, choosing to pay 5 to untap all attacking creatures and to get an additional combat phase after this one.
5. Repeat.