Medomai the Ageless + Idris, Soul of the TARDIS + Thrummingbird + Encroaching Mycosynth

Medomai the Ageless

Idris, Soul of the TARDIS

Thrummingbird

Encroaching Mycosynth

How it works

1. Deal combat damage to an opponent using Medomai the Ageless.
2. Medomai the Ageless triggers, causing you to take an extra turn after this one.
3. Cast Idris by paying 1.
4. When Idris enters the battlefield, it triggers, exiling Medomai the Ageless until Idris leaves the battlefield.
5. Move to your extra turn.
6. At the beginning of your upkeep, Idris's vanishing ability triggers, removing a time counter from it.
7. Deal combat damage to an opponent using Idris and Thrummingbird.
8. Idris and Thrummingbird trigger.
9. Resolve the Idris trigger, causing you to take an extra turn after this one.
10. Resolve the Thrummingbird trigger, putting a time counter on Idris.
11. Repeat from step 5.

This brilliant loop turns Medomai the Ageless into a permanent engine for infinite turns, sidestepping its usual restriction against attacking during extra turns by stealing its soul with Idris, Soul of the TARDIS. Thanks to Encroaching Mycosynth turning every nonland permanent you control into an artifact, Idris can effortlessly imprint Medomai upon entering the battlefield, inheriting the glorious combat-damage trigger of an extra turn without inheriting Medomai's troublesome attack clause. Finally, Thrummingbird swoops in to keep the whole machine alive; whenever it deals combat damage, its proliferate ability adds a time counter back onto Idris, neatly offsetting the vanishing mechanic and securing an endless parade of extra turns.