Go to the first page of the thread and click on "5e Epic Monsters by CR table" you will see an epic bonus row you add that modifier to its ability score modifiers, AC, Attack Bonus, Save DC, and damage to each attack.
Hey Dave, I have some stat blocks that you might or might not want, I have made stat blocks for morgan le fay, merlin, and King Arthur. if you want them I can DM you.
Love the stat block for Acererak that you did! just got the book that holds the tomb of horrors and it wanted me to use the normal stats block for demilich, so thanks for making acererak unique!
All I could find was a lot of lore on him, but have no clue to what any of his stats would be.
this is the link I found.. I hope that helps at all./
https://tolgard.obsidianportal.com/wikis/sessinnek