You have a way to cause a creature to enter the battlefield under your control.
You are not currently venturing in a dungeon.
Steps
Put any nontoken creature onto the battlefield.
When the creature enters,
Radiant Solar,
Moritte of the Frost and
Undercover Operative each trigger.
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.
Resolve the
Moritte of the Frost trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice
Radiant Solar.
Resolve the
Undercover Operative 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
Radiant Solar from your graveyard to the battlefield.
Repeat from step 2.
Notes
If you are currently venturing in a dungeon when starting the combo, you can complete the dungeon and then use
Sefris' second ability to put a creature onto the battlefield to start the combo.
Results
Near-infinite ETB.
Near-infinite LTB.
Near-infinite death triggers.
Near-infinite lifeloss for all players.
Near-infinite sacrifice triggers.
Metadata
In 1 deck according to EDHREC.
Bracket Tag
This combo has been estimated to be Exhibition (Bracket 1+).