You are currently venturing in Tomb of Annihilation in the Trapped Entry room.
Steps
1Cast Hostage Taker by paying its mana cost.
2When Hostage Taker enters, it and Radiant Solar trigger.
3Resolve the Hostage Taker trigger, exiling Cloister Gargoyle until Hostage Taker leaves the battlefield.
4Resolve the Radiant Solar trigger, venturing into the Oubliette room, causing you to discard a card and then sacrifice Hostage Taker.
5Hostage Taker leaves the battlefield, causing Cloister Gargoyle to be returned to the battlefield from exile.
6When Cloister Gargoyle enters, it and Radiant Solar trigger.
7Resolve the Cloister Gargoyle trigger, venturing into the Cradle of the Death God room, creating a 4/4 legendary The Atropal creature token and completing Tomb of Annihilation.
8If you control two or more The Atropal tokens, put all but one into your graveyard due to the legend rule.
9Sefris' second ability triggers, returning Hostage Taker from your graveyard to the battlefield.
10When Hostage Taker enters, it and Radiant Solar trigger.
11Resolve the Hostage Taker trigger, exiling Cloister Gargoyle until Hostage Taker leaves the battlefield.
12Resolve 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.
13Resolve the Radiant Solar trigger from step 6, venturing into the Oubliette room, causing you to discard a card and then sacrifice Hostage Taker.