Progenitor Mimic + Time Stretch + Skullwinder Combo

Time Stretch in hand.

Progenitor Mimic and Skullwinder on the battlefield.

Enough mana available to cast the extra turn spell each turn. Progenitor Mimic is a copy of Skullwinder.

Cast Time Stretch by paying its mana cost, taking an extra turn after this one.

At the beginning of your next upkeep, Progenitor Mimic triggers, creating a token copy of Skullwinder.

When the token Skullwinder enters the battlefield, it triggers itself, returning Time Stretch from your graveyard to your hand.

Repeat.

Infinite turns

Lock