Sefris of the Hidden Ways + Radiant Solar + Progenitor Mimic + Mercurial Pretender

Sefris of the Hidden Ways

Radiant Solar

Progenitor Mimic

Mercurial Pretender

How it works

1. Put any nontoken creature onto the battlefield.
2. When the creature enters, Radiant Solar, Mercurial Pretender and Progenitor Mimic 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 Mercurial Pretender trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice Radiant Solar.
5. Resolve the Progenitor Mimic 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 loop relies on copying a single entering creature to rapidly advance through the Tomb of Annihilation dungeon in a single turn. As you cycle through the rooms, Radiant Solar continually triggers to push you deeper into the dungeon, while Mercurial Pretender and Progenitor Mimic copy the creature to keep the chain moving and eventually generate a torrent of 4/4 The Atropal tokens. Because the legend rule forces you to send extra Atropal tokens straight to the graveyard, Sefris of the Hidden Ways sees those creature cards hit the graveyard and immediately triggers her *Create Undead* ability. That recursion brings Radiant Solar right back to the battlefield, resetting the entire engine to repeat the carnage over and over again.