Ok, we know that the Leveler is evil and that he bends the heroes of every age to his evil ends and...that's about it. A lot of posts wonder how the Leveler is able to control such great heroes. Here's my theory, at least in Connacht's case: Connacht came to HIM!!!
Think about it. In MythTFL the book states that Connacht was the greatest champion of the age, and that he sought out and collected magical artifacts of great power, "destroying those he could, and secreting the rest." Doubtless he did this to protect the realm against future evils. What if, though, he also did this to protect the world from himself?
After Connacht's glorious victories, more power was bestowed upon him than perhaps any person in the Myth world's history (and if I'm wrong on that one, please forgive me--I can't recall verbatim all of the specifics of myth lore, but you get the picture). Not only that, but he had these tremendously powerful artifacts lying about--the Tain, Tramist's Mirror, the Five Eblis Stones, Balmung, the Ibis Crown, etc. At first he destroyed them to rid ensure that their powers would never be abused. Even the strongest mind, though, would eventually be attracted to such artifacts like a moth to a flame (compare what happened with simlar events surrounding the Ring or the Palantiri in The Lord of the Rings). Absolute power corrupts absolutely, and Connacht had absolute power. Maybe he too feared this, and hid the objects for this very reason...
In this way, it was only a matter of time before Connacht came to despise the petty trifles of his citizens or the mundane affairs of a royal court. Connacht depised the weakness of his subjects, their total dependance on him, their claims of equality and liberty. He no longer wanted to rule them; he wanted to destroy them.
With that, Connacht set off to find the very objects that he had secreted so long ago. Armed with such powerful objects, no army could stand against him. Unfortunately, the places where they had been hidden had been long since forgotten, old markers grown over by nature. Maybe it was that he had some of his lieutenants hide the artifacts for him, agents that he was no longer in control of or were deceased. Maybe it was that he secreted them so well that even he himself could not dislodge them from their hiding places. Whatever the case, he didn't have much luck. However, throughout his long search he became aware of an entity that could prove useful in his new endeavor: the Leveler. By allowing the Leveler into his mind, Connacht's mastery of the dark grew, he gained several millenia's worth of knowledge and tactics to use against his enemies. The Leveler would make his ambitions come true...
That, in a nutshell, is my theory. The heroes aren't suddenly possessed by a supreme being, a basic character flaw is necessary to start the transformation process. No matter how powerful these heroes were, being worshipped as a savior of the human race and inheriting a kingdom filled with objects of obscene power would corrupt anyone. Maybe the Leveler calls to these people through such objects, and helps twist their minds (again, the Palintiri), but some part of them must want to assume the mantle before it can happen.
