First Referenced:
The Fountain of Youth
A Multiverse Titan is an entity that lives outside normal space-time, in dimensions where universes can be described as individual bubbles. This space between universes is described as an ocean by the Titan that Ward Shaw (The Sword) connects to inadvertently while its Regeneration Core prepares to emerge and trigger a regrowth of the Titan's full self.
The Multiverse Titan's kind are thought to be extinct, utterly destroyed by the apparently random wild flailing of The Mad God, which also damaged the Outer Shield of human space through a glancing blow, and completely destroyed the Yngtak home universe, along with many others through direct impact.