Sefris of the Hidden Ways + Radiant Solar + Mercurial Pretender + Spark Double

Sefris of the Hidden Ways

Radiant Solar

Mercurial Pretender

Spark Double

How it works

1. Put any nontoken creature onto the battlefield.
2. When the creature enters, Radiant Solar, Spark Double and Mercurial Pretender each trigger.
3. Resolve the Radiant Solar trigger, causing you to venture into Tomb of Annihilation and into the Trapped Entry room, causing each player to lose 1 life.
4. Resolve the Spark Double trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice Radiant Solar.
5. Resolve the Mercurial Pretender trigger, venturing into the Cradle of the Death God room, creating a 4/4 legendary The Atropal creature token and completing Tomb of Annihilation.
6. If you control two or more The Atropal tokens, put all but one into your graveyard due to the legend rule.
7. Sefris' second ability triggers, returning Radiant Solar from your graveyard to the battlefield.
8. Repeat from step 2.

This intricate dungeon loop relies on a clever set of copy effects and recursion to infinitely run through the Tomb of Annihilation. Sefris of the Hidden Ways acts as the engine's heart, watching creature cards hit your graveyard to trigger its powerful completion ability, while Radiant Solar floods the stack with venture triggers whenever creatures enter the battlefield. By having Spark Double and Mercurial Pretender enter as nonlegendary clones of your choice, they amplify those entry triggers and force you deep into the dungeon rooms—culminating in the sacrifice of Radiant Solar and the creation of a legendary Atropal token. The legend rule then neatly drops a duplicate Atropal into the graveyard, sending a creature there to feed Sefris of the Hidden Ways and resurrect Radiant Solar right back to the battlefield to do it all over again.