Preston, the Vanisher | Delney, Streetwise Lookout | Sefris of the Hidden Ways (and one other card)

Cards

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.

This combo has been estimated to be Exhibition (Bracket 1+).