Activate Xenagos's first ability by putting a loyalty counter on it, adding at least {3}{R}{R}{G}{G}.
Cast Wrenn and Six by paying {R}{G}.
Wrenn and Six enters the battlefield, triggering Cloudstone Curio, returning Xenagos from the battlefield to your hand.
Activate Wrenn and Six's second ability by removing a loyalty counter from it, dealing 1 damage to any target.
Cast Xenagos by paying {2}{R}{G}.
Xenagos enters the battlefield, triggering Cloudstone Curio, returning Wrenn and Six from the battlefield to your hand.
Repeat.
Once you have infinite mana, you may instead activate Xenagos' second ability to get infinite creature tokens.

Infinite creature tokens

Infinite damage

Infinite ETB

Infinite green mana

Infinite red mana

Cloudstone Curio

Xenagos, the Reveler

Wrenn and Six

Activate Domri's first ability by putting a loyalty counter on it, adding {R/G}.
Cast Wrenn and Six by paying {0}.
Wrenn and Six enters the battlefield, triggering Cloudstone Curio, returning Domri from the battlefield to your hand.
Activate Wrenn and Six's second ability by removing a loyalty counter from it, dealing 1 damage to any target.
Cast Domri by paying {1}.
Domri enters the battlefield, triggering Cloudstone Curio, returning Wrenn and Six from the battlefield to your hand.
Repeat.

Bard Class

Domri, Anarch of Bolas

Combos with "Wrenn and Six"