Day of the Dragons + Demon of Fate's Design + Enchanted Evening + Cloudstone Curio + Verdant Sun's Avatar

Exhibition

How it works

Cast Day of the Dragons by paying 7 life due to Demon of Fate's Design.
When Day of the Dragons enters the battlefield, its first ability and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, returning nothing to your hand from the battlefield.
Resolve the Day of the Dragons trigger, exiling all creatures you control and creating that many 5/5 Dragon creature tokens.
When the Dragon(s) enter the battlefield, Cloudstone Curio triggers that many times.
Resolve all but one Cloudstone Curio trigger, returning nothing to your hand from the battlefield.
Resolve the final Cloudstone Curio trigger, returning Day of the Dragons to your hand from the battlefield.
When Day of the Dragons leaves the battlefield, its second ability triggers, causing you to sacrifice all Dragons you control and return all cards exiled with Day of the Dragons to the battlefield.
When the creatures that were exiled by Day of the Dragons enter the battlefield, Cloudstone Curio and Verdant Sun's Avatar trigger that many times.
Resolve all Verdant Sun's Avatar triggers, causing you to gain life equal to the collective toughness of the creatures that entered the battlefield.
Resolve all Cloudstone Curio triggers, returning nothing to your hand from the battlefield.
Repeat.

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.

How it works

Cast Day of the Dragons by paying 7 life due to Demon of Fate's Design.
When Day of the Dragons enters the battlefield, its first ability and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, returning nothing to your hand from the battlefield.
Resolve the Day of the Dragons trigger, exiling all creatures you control and creating that many 5/5 Dragon creature tokens.
When the Dragon(s) enter the battlefield, Cloudstone Curio triggers that many times.
Resolve all but one Cloudstone Curio trigger, returning nothing to your hand from the battlefield.
Resolve the final Cloudstone Curio trigger, returning Day of the Dragons to your hand from the battlefield.
When Day of the Dragons leaves the battlefield, its second ability triggers, causing you to sacrifice all Dragons you control and return all cards exiled with Day of the Dragons to the battlefield.
When the creatures that were exiled by Day of the Dragons enter the battlefield, Cloudstone Curio and Verdant Sun's Avatar trigger that many times.
Resolve all Verdant Sun's Avatar triggers, causing you to gain life equal to the collective toughness of the creatures that entered the battlefield.
Resolve all Cloudstone Curio triggers, returning nothing to your hand from the battlefield.
Repeat.