Cast Hostage Taker by paying its mana cost.
When Hostage Taker enters, it and Radiant Solar trigger.
Resolve the Hostage Taker trigger, exiling Underdark Explorer until Hostage Taker leaves the battlefield.
Resolve the Radiant Solar trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice Hostage Taker.
Hostage Taker leaves the battlefield, causing Underdark Explorer to be returned to the battlefield from exile.
When Underdark Explorer enters, it and Radiant Solar trigger.
Resolve the Underdark Explorer 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 Hostage Taker from your graveyard to the battlefield.
When Hostage Taker enters, it and Radiant Solar trigger.
Resolve the Hostage Taker trigger, exiling Underdark Explorer until Hostage Taker leaves the battlefield.
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 Hostage Taker.
Repeat from step 5.

Near-infinite creature ETB

Near-infinite creature LTB

Near-infinite death triggers

Near-infinite lifeloss for all players

Near-infinite creature sacrifice triggers

Radiant Solar

Sefris of the Hidden Ways

Hostage Taker

Underdark Explorer

Cast Fiend Hunter by paying its mana cost.
When Fiend Hunter enters, it and Radiant Solar trigger.
Resolve the Fiend Hunter trigger, exiling Underdark Explorer.
Resolve the 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 Underdark Explorer from exile to the battlefield.
When Underdark Explorer enters, it and Radiant Solar trigger.
Resolve the Underdark Explorer 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 Underdark Explorer.
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.

Fiend Hunter

Complete a dungeon.
Sefris' second ability triggers twice due to Delney.
Resolve the first trigger, returning up to one other creature from your graveyard to the battlefield.
Resolve the second trigger, returning Underdark Explorer from your graveyard onto the battlefield.
When Underdark Explorer enters, it triggers once and Preston triggers twice due to Delney.
Holding priority, activate Yahenni by sacrificing Underdark Explorer.
Resolve a Preston trigger, creating a token copy of Underdark Explorer that is a 0/1 Illusion.
When the Underdark Explorer token enters, it triggers twice due to Delney.
Resolve the Underdark Explorer token triggers, venturing into Undercity twice.
Resolve the second Preston trigger from step 5, creating a token copy of Underdark Explorer that is a 0/1 Illusion.
Repeat steps 8 and 9 one additional time.
Resolve the Underdark Explorer trigger from step 5, venturing into Undercity and completing Undercity.
Repeat from step 2.

Combos with "Underdark Explorer"