Activate Vampire Hexmage by sacrificing it, removing all counters from Invasion of Arcavios.
Invasion of Arcavios triggers, causing you to exile it and cast it as Invocation of the Founders.
Activate Chandra's first ability by putting a loyalty counter on it, adding 1.
Cast Flicker by paying 1, targeting Invocation of the Founders.
Chandra and Invocation of the Founders trigger, copying Flicker twice, with one copy targeting Chandra and the other targeting Shepherd of the Cosmos.
Resolve the first Flicker copy, blinking Chandra.
Resolve the second Flicker copy, blinking Shepherd of the Cosmos.
When Shepherd of the Cosmos enters the battlefield, it triggers, returning Vampire Hexmage to the battlefield from your graveyard.
Resolve the original Flicker, blinking Invocation of the Founders, causing it to enter the battlefield as Invasion of Arcavios.
When Invasion of Arcavios enters the battlefield, it triggers, returning Flicker to your hand from your graveyard.
Repeat.
This intricate loop harnesses the rapid-fire copying power of Chandra, Hope's Beacon and Invocation of the Founders (the back face of Invasion of Arcavios) to turn a simple blink spell into an infinite engine. By sacrificing Vampire Hexmage to instantly flip the Siege without waiting for combat damage, you enable Invocation of the Founders' back-side ability to copy instants and sorceries alongside Chandra's own spell-copying trigger. When you cast Flicker targeting your permanents, these cumulative copy triggers generate enough copies to blink Chandra, Hope's Beacon and Shepherd of the Cosmos simultaneously. That resurrected Shepherd of the Cosmos pulls Vampire Hexmage right back out of the graveyard, resetting the whole cycle once the original Flicker returns to your hand.