First BBEG

[Aurosman]

So the last gaming session my group had we were discussing where we are going in the game and what we would like to do. In the end I thought of an idea for the BBEG when we reach level 20, and talked to the DM about it and he loved it. So much in fact that he told me to create him, or at least most of him. So I am wondering how to go about this, are their any good guides or anyone have any advice?

The BBEG is going to be a former boss of a dungeon from an early dungeon, he was in cahoots with demons so I was thinking of something like having him told he has 1 more chance and they turned him into a demon to give him more power. Would something like this work?

[the_david]

Is he dead? I'm asking because of the way the Great Wheel/Planescape Cosmology works.
In general, if you are a chaotic evil demonworshipper you'd end up in the abyss as a petitioner. (Just like you'd end up in hell if you'd sell your soul to the devil.) From there you can go on to become a demon and work your way up by backstabbing and eating lots of damned souls. Becoming a Balor level demon would probably take centuries though.
It also doesn't happen very often that an outsider remembers its past life. Some memories might stick, but usually you'll lose your memory with every transformation. It could be possible that your BBEG holds on to those few memories of his defeat and develops a grudge, but I don't think that's very likely.

[Inevitability]

This fits the fluff of the Soul Eater class reasonably well, and as far as villainous PrCs go you could definitely do worse.

You'd need a way to give the BBEG lots of attacks per turn, though, preferably through natural weapons.

[Karl Aegis]

It's just a demon. I don't see anything that makes it a special demon.

[flappeercraft]

I would make it any powerful Demon but with his class levels of his previous life added on top. A Balor with class levels could be pretty terrifying if well optimized.

[Waker]

The BBEG is going to be a former boss of a dungeon from an early dungeon, he was in cahoots with demons so I was thinking of something like having him told he has 1 more chance and they turned him into a demon to give him more power. Would something like this work?

What were some of the villains traits pre-death? Were they a mage, warrior type or something else? Building off of that may help the party recognize the character better. As others have said, the process of transforming a damned soul into a demon/devil is a lengthy process. You might instead consider having him become an undead with a bit of fiendish flavor. Something like a Lich with Fire immunity instead of Cold immunity and a focus on summoning more demons as an example.

[syryous]

What were some of the villains traits pre-death?

The bad guy in question was Gashkarr from The Shattered Gates of Slaughterguarde. So... Gnoll Ranger 5/Blackguard 3.

[Gildedragon]

Two words: Hell Bred
Keep the class, boost that, have them be a soul that clawed their way out of the abyss tooth and nail.

Turn hellbred into a template

Alternatively

Pick an undead template
Spellstitch him with spiteful spells

[John Longarrow]

Former boss? Did he survive you cleaning out the dungeon?

What was he when he was a boss?

[Aurosman]

Yeah it was the guy syryous said. We basically still have the tome of slaughter guard since we can't destroy it. So we put it in a non scryable, guarded, etc, etc vault in the shining citadel with the luminous order guarding it. Only to have the ebon cabal steal it somehow and open a portal to study the abyss only to have the guy come out and start wrecking havok.