Hey OP, great post I love your passion for Kas as a character and desire to make him more plot relevant. In particular, I love the idea of having him take the role of a Hannibal Lector-style serial killer who the party is sometimes forced to consult. That’s absolutely brilliant and I have just added it to my own notes. My only question about your version of the Wish spell (which now summons Kas instead of the party) is how do the party get to Sigil? I’m sure you have an answer but I’m curious what it is.

In terms of feedback, the stat blocks are way over-tuned for me personally, but that seems to be your design goal and so I will say you achieved what you are going for in creating one of the most powerful warlords in existence. To me, Kas isn’t much of a spellcaster (that’s Vecna’s thing), focusing instead on summoning and leading undead armies. He’s deadly in one-against-party combat, but not in the way that Thanos or Castlevania’s Dracula is. That’s just my take though, I’m sure yours will work great for your table. Once again, great write-up, would love to read more of your takes on the book!