Mirrormade + Estrid's Invocation + Oblivion Ring + Altar of the Brood

Mirrormade

Estrid's Invocation

Oblivion Ring

Altar of the Brood

How it works

1. Cast Mirrormade by paying 2.
2. Mirrormade enters the battlefield as a copy of Oblivion Ring, triggering Altar of the Brood and itself.
3. Resolve the Altar of Brood trigger, causing each opponent to mill a card.
4. Resolve the Mirrormade's trigger, exiling Oblivion Ring.
5. Cast Estrid's Invocation by paying 2.
6. Estrid's Invocation enters the battlefield as a copy of the Oblivion Ring copy, triggering Altar of the Brood and itself.
7. Resolve the Altar of Brood trigger, causing each opponent to mill a card.
8. Resolve the Estrid's Invocation's trigger, exiling Mirrormade.
9. Mirrormade leaves the battlefield, triggering itself, returning Oblivion Ring from exile to the battlefield.
10. Altar of the Brood and Oblivion Ring trigger.
11. Resolve the Altar of Brood trigger, causing each opponent to mill a card.
12. Resolve the Oblivion Ring trigger, exiling Estrid's Invocation.
13. Estrid's Invocation leaves the battlefield, returning Mirrormade from exile to the battlefield.
14. Repeat from step 2 until each opponent has an empty library, then end the loop by choosing to copy or exile any other permanent.