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 Abdel Adrian from the command zone by paying {4}{W} and command tax, if applicable.
When Abdel Adrian enters the battlefield, it triggers, exiling Hostage Taker and creating a 1/1 Soldier creature token.
When Hostage Taker leaves the battlefield, Felidar Guardian returns from exile to the battlefield.
When Felidar Guardian enters the battlefield, it triggers, blinking Abdel Adrian.
Abdel Adrian leaves and re-enters the battlefield, causing Hostage Taker to return from exile to the battlefield, and triggering both Hostage Taker and Abdel Adrian's abilities.
Resolve the Hostage Taker trigger, exiling Felidar Guardian until Hostage Taker leaves the battlefield.
Resolve the Abdel Adrian trigger, exiling Hostage Taker and creating a 1/1 Soldier creature token.
Repeat from step 3.

Infinite creature LTB

Infinite creature ETB

Infinite creature tokens

Infinite blinking of nonland permanents

Abdel Adrian, Gorion's Ward

Felidar Guardian

Activate Kiki-Jiki by tapping it, creating a token copy of Hostage Taker.
When the Hostage Taker token enters the battlefield, it triggers, exiling Kiki-Jiki until Hostage Taker leaves the battlefield.
Activate Ashnod's Altar by sacrificing the Hostage Taker token, adding {C}{C}.
When the Hostage Taker token dies, Kiki-Jiki is returned from exile to the battlefield.
Repeat.

Infinite creature sacrifice triggers

Infinite death triggers

Infinite colorless mana

Kiki-Jiki, Mirror Breaker

Ashnod's Altar

Cast Hostage Taker by paying {2}{U}{B}.
Hostage Taker enters the battlefield, exiling Angel of Glory's Rise.
Activate Altar of Dementia by sacrificing Hostage Taker.
Hostage Taker leaves the battlefield, returning Angel of Glory's Rise from exile to the battlefield.
Angel of Glory's Rise enters the battlefield, returning Hostage Taker from your graveyard to the battlefield.
Repeat from step 2.
Once you have a large amount of Altar of Dementia abilities on the stack, you may stop the loop and resolve them to cause target player(s) to mill infinite cards.

Infinite mill

Infinite self-mill

Angel of Glory's Rise

Altar of Dementia

Cast Hostage Taker by paying {2}{U}{B}.
Hostage Taker enters the battlefield, exiling Angel of Glory's Rise.
Activate Ashnod's Altar by sacrificing Hostage Taker, adding {C}{C}.
Hostage Taker leaves the battlefield, returning Angel of Glory's Rise from exile to the battlefield.
Angel of Glory's Rise enters the battlefield, returning Hostage Taker from your graveyard to the battlefield.
Repeat from step 2.

Cast Hostage Taker by paying {2}{U}{B}.
Hostage Taker enters the battlefield, exiling Angel of Glory's Rise.
Activate Phyrexian Altar by sacrificing Hostage Taker, adding one mana of any color.
Hostage Taker leaves the battlefield, returning Angel of Glory's Rise from exile to the battlefield.
Angel of Glory's Rise enters the battlefield, returning Hostage Taker from your graveyard to the battlefield.
Repeat from step 2.

Infinite colored mana

Phyrexian Altar

Combos with "Hostage Taker"