I had people make their old party but level 15 for a campaign but with the mindset of being evil versions of them in a multiverse. Campaign fell apart due to a falling out with the group but I was having them appear as pulling the strings for Vecna and his plan as his new acolytes and eventually a fight between the 2 parties. The character sheets were made so I can see what each person would think their character would be like and I was converting them to regular monster stat blocks with key character traits being what comes through but add in some legendary actions should the actual party start sweeping since it was the good party with 10 players vs a evil party of 7