#for i was cheating on you
You form the party out of normal adventurers who met in the bar, all of whom have some reason to hate the BBEG. This is something each of them needs to figure out for themselves.
Then midway through the campaign, they find out who the BBEG used to be before s/he Ascended. And then as the DM, you tell them they are all shocked by this news, and pass them all secret envelopes while explaining how all of them have a backstory with the guy that haunts them and that they don’t want to talk about. Imply that there are a wide range of relationships (sibling who went bad, bad boy at university who almost tempted them into something heinous, etc.)
However, all that done, each of them reads their secret letter and finds out independently that they were the “one” dating the BBEG. Let them come up with the ways in which they hated the relationship and broke it off (“I was the one who turned him bad, before I changed my tune” would be great, for example), but do give them a timeline for when they dated.
For at least two of the party, give them overlapping dates, and information that they were super suspicious that they were being cheated on, although they could never prove it, and that’s part of what woke them up to the bad relationship.
If you play it right, you can keep dropping hints to other types of relationships that you haven’t set up, right up until the final confrontation.
When the BBEG is ready to face a group of plucky heroes, and is shocked that they managed to gather every one of his evilgood exes in one place.
