Medomai the Ageless + Idris, Soul of the TARDIS + Karn's Bastion + Biotransference

Medomai the Ageless

Idris, Soul of the TARDIS

Karn's Bastion

Biotransference

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. Activate Karn's Bastion's second ability by paying 4 and tapping it, putting a time counter on Idris.
8. Deal combat damage to an opponent using Idris.
9. Idris triggers, causing you to take an extra turn after this one.
10. Repeat from step 5.

This crafty infinite turn loop gets around Medomai the Ageless's restriction against attacking during extra turns by turning to Idris, Soul of the TARDIS instead. Thanks to Biotransference making creatures into artifacts, you can cast Idris to imprint Medomai the Ageless, granting Idris the legendary sphinx's triggered ability to take extra turns upon dealing combat damage. Because Idris is a fresh entity that didn't take the extra turn itself, it bypasses Medomai's clause and can safely swing for combat damage turn after turn. To keep the TARDIS from vanishing away forever, Karn's Bastion steps in to proliferate and continuously replenish the time counters on Idris during each upkeep.