Medomai the Ageless + Idris, Soul of the TARDIS + Thrummingbird + Ashnod's Transmogrant

Medomai the Ageless

Idris, Soul of the TARDIS

Thrummingbird

Ashnod's Transmogrant

How it works

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

This loop bypasses Medomai the Ageless's restriction against attacking during extra turns by turning it into an artifact with Ashnod's Transmogrant and exiling it under Idris, Soul of the TARDIS as it enters the battlefield. Because Idris inherits Medomai's combat-damage trigger while retaining its own creature type, it can safely swing for infinite extra turns. Meanwhile, Thrummingbird swoops in on every combat step to proliferate, placing a time counter back onto Idris to reset its vanishing countdown and keep the loop running indefinitely.