Cast Hostage Taker by paying its mana cost.
When Hostage Taker enters, it and Radiant Solar trigger.
Resolve the Hostage Taker trigger, exiling Barrowin of Clan Undurr 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 Barrowin of Clan Undurr to be returned to the battlefield from exile.
When Barrowin of Clan Undurr enters, it and Radiant Solar trigger.
Resolve the Barrowin of Clan Undurr 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 Barrowin of Clan Undurr 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

Barrowin of Clan Undurr

Cast Fiend Hunter by paying its mana cost.
When Fiend Hunter enters, it and Radiant Solar trigger.
Resolve the Fiend Hunter trigger, exiling Barrowin of Clan Undurr.
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 Barrowin of Clan Undurr from exile to the battlefield.
When Barrowin of Clan Undurr enters, it and Radiant Solar trigger.
Resolve the Barrowin of Clan Undurr 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 Barrowin of Clan Undurr.
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 Barrowin of Clan Undurr from your graveyard onto the battlefield.
When Barrowin of Clan Undurr enters, it triggers once and Preston triggers twice due to Delney.
Resolve a Preston trigger, creating a token copy of Barrowin of Clan Undurr that is a 0/1 Illusion.
When the Barrowin of Clan Undurr token enters, it triggers twice due to Delney.
Resolve the first Barrowin of Clan Undurr token trigger, venturing into Tomb of Annihilation and into the Trapped Entry room, causing each player to lose 1 life.
Resolve the second Barrowin of Clan Undurr token trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice the nontoken Barrowin of Clan Undurr.
Resolve the second Preston trigger from step 5, creating a token copy of Barrowin of Clan Undurr that is a 0/1 Illusion.
When the Barrowin of Clan Undurr token enters, it triggers twice due to Delney.
Resolve the first Barrowin of Clan Undurr token 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.
Repeat from step 2.

Combos with "Barrowin of Clan Undurr"