I'm going to be starting a pretty short campaign here soon. I've mostly got the plot figured out, and when I was helping a player with character creation they asked if there was a good patron for their Warlock character. My initial thought was, "well the BBEG would be interesting."

The players are just exploring a ruined castle/island that's been twisted and it's inhabitants warped due to exposure to a great old one- they're trying to solve the problem and make the place habitable again.

The warlock's patron is the BBEG, an eldritch god trying to force his way onto their plane. He's currently held out of the plane by a magical seal that can't be broken by him or any of his servants. So the Warlock player needs to- subtly- get the party all the way to the seal and manipulate them into breaking it. I did this specifically to avoid him murder hoboing his party members. They have different goals, but they still need each other.

If this goes badly, I figure the worst case scenario is that he gets slapped by the other 5 party members. So I've asked him to have at least an idea for a second character.