Kotis, Sibsig Champion + Altar of Dementia + Skullwinder

Kotis, Sibsig Champion

Altar of Dementia

Skullwinder

How it works

1. Cast the extra turn spell by paying its mana cost, causing you to take an extra turn after this one.
2. Move to your extra turn.
3. Activate Altar of Dementia by sacrificing Skullwinder, causing you to mill cards equal to Skullwinder's power.
4. Cast Skullwinder from your graveyard by paying its mana cost and exiling three other cards from your graveyard, ensuring not to exile the extra turn spell.
5. When Skullwinder enters the battlefield, it triggers, returning the extra turn spell from your graveyard to your hand.
6. Repeat.