Cast Glasspool Mimic by paying {0}, choosing to have it enter the battlefield as a copy of Barrin.
When Glasspool Mimic enters the battlefield, its first ability triggers, returning Barrin from the battlefield to your hand.
Cast Barrin by paying {0}.
When Barrin enters the battlefield, its first ability triggers, returning Glasspool Mimic from the battlefield to your hand.
Repeat.

Infinite creature ETB

Infinite creature LTB

Infinite storm count

Sakashima of a Thousand Faces

Aluren

Barrin, Tolarian Archmage

Glasspool Mimic // Glasspool Shore

Cast Mirror Image by paying {0}, choosing to have it enter the battlefield as a copy of Barrin.
When Mirror Image enters the battlefield, its first ability triggers, returning Barrin from the battlefield to your hand.
Cast Barrin by paying {0}.
When Barrin enters the battlefield, its first ability triggers, returning Mirror Image from the battlefield to your hand.
Repeat.

Mirror Image

Mirror Gallery

Mirror Box

Combos with "Barrin, Tolarian Archmage"