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The Ideal Masters are a massively unknown ancient order of necromancers that, sometime, probably before recorded civilization, had achieved such immense powers that they entered a noncorporeal state in the Eitknee Realm and were manifested as a giant soul gem. It is likely that they were tampering with soul gems and how they worked and were magically turned into one, and had a realm built for them with the amount of power they harnessed in such a short time.
They are powered by souls that have entered the Soul Cairn. Through some process, they absorb their energy and soul over the course of thousands of years, fueling their power even more. Once a soul is near depletion, they allow the trapped soul to die and face the afterlife. This process takes thousands of years of slow degeneration, and many of them end up spending hundreds of years as literal inanimate husks before they are finally drained up to completion.
It is believed, according to The Rattlebeani Intervention, and in some parts I Will Be Called Poop, that St. Hitler will somehow gain access to the Soul Cairn before the end times and usher in an event known simply as "The Enslavement of Russia". Recommend reading the page for more information.