Slaaneshi Chaos Sorc for WFRP themed D&D

[KillingTime]

As it says in the title.
A friend of mine and I are statting up a Slaaneshi Chaos Sorcerer for an Old World themed campaign. He (she? they?) is the BBEG for the campaign.
A group of six character will be at level 9 or 10 by the time they hit the boss fight.
Rogue/Ranger/War Cleric/Barbarian/Dragon Sorc/Fighter

Currently we're looking at building them as a high level PC.
We've got some ideas on how to stat them, but I'm very interested to see how the playground would approach it.

Gimme classes, spell choices, level.
All books, Homebrew, UA - it's all on the table.
Except 1 level Hexblade dips. Anyone suggesting this needs to go home and think about their life choices.

[Xervous]

As much as I detest LOLrandom this is one of the few cases I’d advise having a wonderfully painful random table to keep the spirit of Psykers and Ruinous Powered spellcasters.

Also, two level hexblade dip.

[KillingTime]

As much as I detest LOLrandom this is one of the few cases I’d advise having a wonderfully painful random table to keep the spirit of Psykers and Ruinous Powered spellcasters.

I think at least 1 level of Wild Magic Sorcerer is pretty much given.

[JackPhoenix]

It's an NPC. No class, and therefore no level, otherwise, you'll be disappointed with the result, 5e is not build for PvP. CR whatever you want, depending on how difficult is the fight supposed to be, and if the BBEG will be alone or if it'll have support, but you're propably looking at something between 15-20, stats as appropriate for the CR at hand, likely focused Cha and more on Dex than Str thematically, not much in the way of armor. Spells focused on enchantment, illusions and general mind**ckery, summonig daemons would also be appropriate. You can use Wild Magic table for the chaotic randomness without involving sorcerer class in any way, or you can try google search for something with less positive effects.

Lazy choice, take Graz'zt from MToF and make him squishier, his damage already isn't that great, and he's pretty appropriate otherwise.