Nah, Kas doesn't need the rod, he has been fed the line by Tasha. He isn't a wizard, he doesn't really know any more than the PCs (and wouldn't be able to subvert Vecna's ritual in any case). It's Iggwilv the demonologist who is allied with Miska and wants to subvert the unlimited powah ritual for herself.
The rod imprisoned Miska, so it can free him, but it can also be used for other things, especially when it looks like chaos is getting the upper hand in the multiversal balance.
The rod is yet another D&D artifact based on Moorcock, in this case, the Runestaff, which is the lawful equivalent of chaotic Stormbringer (which is Blackrazor in D&D).
I was thinking of making Kas's crown amber, or at least have amber in it, to make it more clearly a vessel for the Dark Powers. If Kas steps out of line it has the power to immediately return him to Ravenloft.