Lumra, Bellow of the Woods + Mirrorpool + Troubled Healer + Horizon Explorer

Lumra, Bellow of the Woods

Mirrorpool

Troubled Healer

Horizon Explorer

How it works

1. Activate any number of other lands by tapping them, adding at least 4C.
2. Activate Troubled Healer by sacrificing a land except Mirrorpool.
3. Repeat step 2 until you control no lands except Mirrorpool.
4. Activate Mirrorpool's third ability by paying 4C, tapping and sacrificing it, creating a token copy of Lumra.
5. Put the Lumra token into your graveyard due to the legend rule.
6. The Lumra token's enters-the-battlefield ability triggers, causing you to mill four cards and return all land cards from your graveyard to the battlefield untapped due to Horizon Explorer.
7. Repeat.

This intricate loop leverages the sacrifice outlets and recursive power of your board to generate an infinite engine. By clearing away your lands with Troubled Healer, you set up the exact conditions needed for Mirrorpool to copy Lumra, Bellow of the Woods. Because Horizon Explorer ensures your lands enter untapped, every iteration of the Lumra token's enter-the-battlefield trigger returns your entire graveyard of lands straight to the battlefield to fuel the loop anew, while milling your deck in the process.