Self-centered, possibly, but arrogant is a matter of opinion. With the caveat that this discussion is attempting to impose order upon the mystical ramblings of a game writer blasted out of his mind on mushrooms, you really only have to be able to retain personhood while witnessing your universe from the outside.
In other words, any character who breaks the fourth wall while retaining their character has arguably achieved CHIM, and the zero summed are those whose straddling of the suspension of disbelief undermines their nature and prevents their inclusion in the story. Vivec, that cheat, scammed his way through with help from the Heart of Lorkhan and a hint from Morgoth Bal, but you don't necessarily have to be full of yourself to manage it.
Anyway. That's not the threat. Dragon priests aren't the threat.
The threat is that two D&D spellcasters who commit to necromancy and infernalism think they know what they're doing with summoning up devils and making contracts. Maybe they do!
They're not prepared for the daedra, and the daedra have absolutely everything they've ever wanted. Mei Wen's going to need every advantage her cultivation gives her if her mother and father decide to compact with Coldharbor, and might even need her pet goat's help if Sheogorath gets involved. In Daggerfall, every time you summon up a daedric prince, there's a 5% chance he'll show up instead!