Kas for Vecna Campaign

I'm running a campaign that takes place before Vecna becomes a god, and I wanted to have Kas as both an enemy and 'ally' to the players, as he seeks to kill Vecna as well. The problem is, I'm stuck on how I should make him. Should I make him a Death Knight? A vampire? A Vampire Death Knight? Looking for suggestions.

It depends how close to 'lore' you want to follow but Kas started out as a paladin and so you might want to build him as a high level Oathbreaker.  There's some debate as to whether he became a vampire during or after his time with Vecna but perhaps you could build this in as a plot point, having him turn during your campaign. He became a vampire through a mysterious ritual involving a mask so,again, it's up to you which vampiric attributes you give him. So, yeah, I'd build him as an Oathbreaker with vampiric attributes.

I'm curious to know, are you running him with Kas' sword from the DMG, or are you homebrewing that as well?

I'm running him with Kas' sword, and I was debating on adding the Death Knights extra necrotic damage to it since trying to calculate the CR of a creature is a pain if you give it the paladin smite. Though I'm not sure if I want to add the DK necro damage so I can avoid going over a certain CR threshold (between 20 and 22)

Obviously, he'll have legendary actions (except for the final encounter with Vecna in order to keep things fair for the party), although I'm not sure if I should stick to the vampire's legendary actions or give Kas his own, as I'm trying to, again, avoid going over a specific CR threshold.

Speaking of Vampires, I'm still trying to figure out which vampire stat to use, either the vampire warrior or the blood-drinker vampire.

Kas should only ever be a Deathknight of a sort, or simply Oathbreaker.  The vampire angle is well, very WoTClike.

Quote from OccultCenturion >>

I'm running him with Kas' sword, and I was debating on adding the Death Knights extra necrotic damage to it since trying to calculate the CR of a creature is a pain if you give it the paladin smite. Though I'm not sure if I want to add the DK necro damage so I can avoid going over a certain CR threshold (between 20 and 22)

Obviously, he'll have legendary actions (except for the final encounter with Vecna in order to keep things fair for the party), although I'm not sure if I should stick to the vampire's legendary actions or give Kas his own, as I'm trying to, again, avoid going over a specific CR threshold.

Speaking of Vampires, I'm still trying to figure out which vampire stat to use, either the vampire warrior or the blood-drinker vampire.

 For sure use warrior, it makes more sense cause he's canonically a general of Vecna's army.