Cast Corpse Dance with buyback by tapping Caetus, Vampire Hexmage and three other creatures you control.
Displacer Kitten triggers, blinking Caetus and causing it to return to the battlefield as Invasion of Segovia.
When Invasion of Segovia enters the battlefield, it triggers, creating four 1/1 Kraken creature tokens.
Activate Vampire Hexmage by sacrificing it, removing all counters from Invasion of Segovia.
Invasion of Segovia triggers, causing you to exile it and cast it as Caetus, Sea Tyrant of Segovia.
Resolve Corpse Dance, returning Vampire Hexmage from your graveyard to the battlefield and returning Corpse Dance to your hand.
Repeat.
This intricate loop relies on looping Corpse Dance with buyback, fueled entirely by the convoke granted by Invasion of Segovia (the back face of Invasion of Segovia // Caetus, Sea Tyrant of Segovia) to cast it without spending traditional mana. Each time you cast the spell, Displacer Kitten triggers to blink Caetus back into its un-transformed Siege state, generating a fresh batch of tokens—doubled instantly by Mondrak, Glory Dominus to provide plenty of creatures for the next convoke payment. You then use Vampire Hexmage's sacrifice ability to strip the defense counters off the Siege immediately, triggering its transformation back into Caetus while Corpse Dance simultaneously plucks Vampire Hexmage right back out of the graveyard to restart the whole sequence.