My inclination is to simply say, don't stat him. If you give him stats that implies that the PCs can find a way to beat him in a straight up fight and I just don't see that happening. What I would suggest considering is to handle it in the same way a bunch of other of Arcanis' epic stories have been resolved. The PCs are critical to getting pieces in place and tipping the balance in the favor of the outcome that they're looking for.
If you went this way, arguably the PCs over time would need to determine who/what is capable of defeating the SK, a being who is immortal, who imprisoned an Avatar, helped experiment on a Valinor and used that to experiment on himself, who took on all the Myrantian Necromancer Lords, the Eurynellian Mages, etc. I would think it would take multiple True Dragons / Valinors to have a chance which raises the question of what's in it for them. ALTERNATIVELY if you wanted to do it with a twist, the SK seeks godhood, claiming all arcane casters are his domain. If he achieved his goal, you could argue he'd leave the mundane world behind to take his proper place in the universe, called to help Illir and the others deal with threats that they alone are powerful enough to have a hope against. Arguably he'd leave a priesthood behind, but if he succeeded he wouldn't need those he'd enslaved anymore. This opens up a lot of adventures around steps, information and items needing to achieve godhood with the expectation that they will be carefully and jealously guarded.
In terms of actual mechanics, I'm not a 5E guy, so I'll defer to Eric and others to provide reasonable guidance. Just thought I'd provide some alternative means and stories to an end.
With a sweep of his hat,
Paul
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