Need Ideas for a Level 20 Oneshot

So, I had the briliant idea to propose to run a level 20 oneshot for some of my friends. But it turns out, level 20 D&D is really hard to prep. 

So, I managed to created the introduction where the players will face a couple of puzzles and some light combat where they can test their new abilities a bit out before confronting the BBEG. Now the problem is, I don't have a BBEG yet...

So if anyone has any good suggestions for a BBEG that I can use against 5 lvl 20 characters, or knows of any good published (official or 3rd party) level 20 oneshots that I can look to for inspiration, please share them. I would love to hear all of your ideas.

if you are really bored, give them a valor bard warlock. conjure minor elementals+eldritch blast. (chefs kiss)

Sounds fun, but unfortunately we agreed to use the 2014 rules only. So I might just have to use a gloomstalker/rogue/fighter multiclass instead ;)

Quote from EarthSword28 >>

Sounds fun, but unfortunately we agreed to use the 2014 rules only. So I might just have to use a gloomstalker/rogue/fighter multiclass instead ;)

 sorry I don't know much 2014.