Radiant Solar trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice
Fiend Hunter.
When
Fiend Hunter dies, it triggers, returning
Undermountain Adventurer from exile to the battlefield.
When
Undermountain Adventurer enters, it and
Radiant Solar trigger.
Resolve the
Undermountain Adventurer trigger, venturing into the Cradle of the Death God room, creating a 4/4 legendary The Atropal creature token and completing Tomb of Annihilation.
If you control two or more The Atropal tokens, put all but one into your graveyard due to the legend rule.
Sefris' second ability triggers, returning
Fiend Hunter from your graveyard to the battlefield.
When
Fiend Hunter enters, it and
Radiant Solar trigger.
Resolve the
Fiend Hunter trigger, exiling
Undermountain Adventurer.
Resolve the
Radiant Solar trigger from step 10, venturing into Tomb of Annihilation and into the Trapped Entry room, causing each player to lose 1 life.
Resolve the
Radiant Solar trigger from step 6, venturing into the Oubliette room, causing you to discard a card and then sacrifice
Fiend Hunter.
Repeat from step 5.
Results
Near-infinite ETB.
Near-infinite LTB.
Near-infinite death triggers.
Near-infinite lifeloss for all players.
Near-infinite sacrifice triggers.
Metadata
In 6 decks according to EDHREC.
Bracket Tag
This combo has been estimated to be Exhibition (Bracket 1+).