Cergorach
The Laughing One
Why bother changing that much? The actual PCs got a premonition via one of their hag allies that a party of heroes would collect the Rod of Seven Parts to thwart Vecna. They killed the hag, so it couldn't repeat the premonition and they murdered the heroes and took their place, pulling a 'double-Kas'. Just run the adventure as is and now the PCs get a turn to betray Kas... The PCs being evil masterminds will of course want to eliminate high level competition, be it 'good' or 'evil'... Maybe even making some deals with devils in Avernus along the way...