Preston, the Vanisher + Delney, Streetwise Lookout + Sefris of the Hidden Ways + Barrowin of Clan Undurr

Decks

How it works

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.

In which brackets is this combo allowed, per Spellbook's Exhibition tier?

1–2 · Exhibition & Core

Allowed in this bracket.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.