Activate Ultimecia, Time Sorceress using the other transforming creature's ability by paying its activation cost, transforming Ultimecia, Time Sorceress into Ultimecia, Omnipotent.
Ultimecia, Omnipotent triggers, causing you to take an extra turn after this one.
Activate Ultimecia, Omnipotent using the other transforming creature's ability by paying its activate cost, transforming Ultimecia, Omnipotent into Ultimecia, Time Sorceress.
Repeat each turn.

Infinite turns

Creature with "Pay Mana: Transform this creature"

Ultimecia, Time Sorceress // Ultimecia, Omnipotent

Agatha's Soul Cauldron

Cast Ultimecia, Time Sorceress from your hand or from your graveyard by paying {3}{U}{B}.
When Ultimecia, Time Sorceress enters the battlefield, it triggers, causing you to surveil 2.
At the beginning of your end step, Ultimecia, Time Sorceress triggers, causing you to pay {4}{U}{U}{B}{B} and exile eight cards from your graveyard, transforming Ultimecia, Time Sorceress into Ultimecia, Omnipotent.
Ultimecia, Omnipotent triggers, causing you to take an extra turn after this one.
Activate Altar of Dementia by sacrificing Ultimecia, Omnipotent, causing you to mill seven cards.
Re

Near-infinite turns

Muldrotha, the Gravetide

Altar of Dementia

Activate Isochron Scepter by paying {2} and tapping it, casting a copy of Moonmist without paying its mana cost.
Resolve Moonmist, transforming Ultimecia, Time Sorceress into Ultimecia, Omnipotent.
Ultimecia, Omnipotent triggers, causing you to take an extra turn after this one.
At the beginning of your end step, Thassa triggers, blinking Ultimecia, Omnipotent and causing it to re-enter the battlefield as Ultimecia, Time Sorceress.
Whenever Ultimecia, Time Sorceress enters the battlefield, it triggers, causing you to surveil 2.
Repeat each turn.

Moonmist

Isochron Scepter

Thassa, Deep-Dwelling

At the beginning of your upkeep, Panoptic Mirror triggers, casting a copy of Moonmist without paying its mana cost.
Resolve Moonmist, transforming Ultimecia, Time Sorceress into Ultimecia, Omnipotent.
Ultimecia, Omnipotent triggers, causing you to take an extra turn after this one.
Deal combat damage to an opponent using Brago, triggering Brago, blinking Ultimecia, Omnipotent and causing it to re-enter the battlefield as Ultimecia, Time Sorceress.
Whenever Ultimecia, Time Sorceress enters the battlefield, it triggers, causing you to surveil 2.
Repeat each turn.

Combos with "Ultimecia, Time Sorceress // Ultimecia, Omnipotent"