Abyssal god dragon

[tonyblitz1]

trying to construct a "mad" black Dragon as a near apocalyptic enemy for a high level campaign. Most players are nearing level 30. With clever tactics, smart deals with greater powers and seemingly blind luck, the party has killed/defeated/banished no less than 5 deities.
As part of an ongoing story a very old very powerful black Dragon has been awakened, and the concept is that it has been staring into the abyss for centuries as it slept, and taken this power (and insanity) becoming a being of absolute chaotic evil wrath.

Near as I can tell it would require giving the Dragon class levels in something really nasty as I can't find any good templates for abyssal creatures.

but I've never constructed a custom creature such as this and don't know where to start.

I should also add that psions and anything from pathfinder are disallowed in this campaign.
Otherwise any 3.5 source is fair game, including regional.

[Fayd]

First, what system are you using?

Regardless, have you considered giving it Shadowdancer levels? The idea of a creature with At Will (in Pathfinder anyway) Darkness and the ability to teleport between shadows, it could be a surprisingly mobile creature, and a Shadow based on a dragon could be a truly frightening creature. It's not quite fiendish, but it'd make that dragon pretty tough.

[tonyblitz1]

Would Dragon Ascendant be a good starting point?

[MisterKaws]

Make it an Advanced Dragon with eleven additional age categories.

Give it 20 Divine Ranks.

Salient Divine Abilities: Alter Reality, Alter Size, Alter Form, Shift Form, Shapechange, True Shapechange, Instant Move(all of this for the extremely chaotic nature), Divine Dodge, Instant Counterspell, and then eleven Automatic Metamagic feats, chosen by yourself in the cheesiest combination possible.

I'm also pretty sure that emulating Psionics with Alter Reality is fine, so just cheese Metafaculty to let him know all the PC's moves. He does have the ability to sense anything related to him that might possibly happen in the next five months, too.

[Draconium]

Would Dragon Ascendant be a good starting point?

Eh, most of what Dragon Ascendant does is giving the dragon the abilities of Divine Rank 0, without actually giving them a Divine Rank. That, and some nifty changes to their Frightful Presence. I would skip it and just go with the Divine Ranks.

[Flickerdart]

When in doubt, Epic templates like pseudonatural (http://www.d20srd.org/srd/epic/monsters/pseudonaturalCreature.htm) or paragon (http://www.d20srd.org/srd/epic/monsters/paragonCreature.htm) are a good enough patch.

[DrMotives]

Make it a Chole dragon, those are insane dragons native to the Abyss.

[tonyblitz1]

What I've been looking at to play around with is
psudonatural (outsider)
Ritual of alignment (evil)

Fiend of possession,
And then I can really have fun tormenting the party with him.

[Arcane_Secrets]

trying to construct a "mad" black Dragon as a near apocalyptic enemy for a high level campaign. Most players are nearing level 30. With clever tactics, smart deals with greater powers and seemingly blind luck, the party has killed/defeated/banished no less than 5 deities.

I should also add that psions and anything from pathfinder are disallowed in this campaign.
Otherwise any 3.5 source is fair game, including regional.

That's fine. Before I get into more specific ideas could you be a bit more specific as to what (if anything) this particular dragon wants? Does it just want to destroy everything which means that it'd have different abilities than if it had some particular obsession like turning your world into acid so it has a nice place to swim forever?

[Crake]

Does it have to be specifically a black dragon? Or just a black colored dragon? I'm quite fond of tarterian dragons personally. I also quite like dragon ascendant, but then, I also religiously (ironic choice of words) avoid the divine rank rules, because really, anyone with divine ranks should be completely untouchable by someone without divine ranks, and if they do it just seems so unbelievable that for me personally it feels completely unnatural and breaks any sense of verisimilitude.

A great wyrm dragon ascendant tarterian dragon with epic spellcasting, wyrm of war, and spellhoarding psychosis, who's gone through and exchanged all his non-epic feats for epic feats via the dark chaos shuffle, has access to practically every spell in existance at his whim by having multiple copies in his spellhoard, and a metric ton of epic feats, preferably including all the automatic metamagics you could ever want seems like it should be enough of a challenge for your party of level 30s, though honestly when you get to those kinds of levels, the casting shenannigans just make them game rocket tag, and it's just a question of who gets the first action in, because it only takes 1 action to be able to win the fight.

[tonyblitz1]

The Dragon revels in causing chaos and panic, and wants everyone to see the abyss and eventually turn every plane into an abyssal plane.
I have been allowed a divine rank of 11.
He has also been homebrewed the ability to show the abyss to people who meet his gaze, and cause flashing glimpses of it to occur in people's dreams. He enjoys possessing people for the purpose of random murder and destruction.

[Clistenes]

You could just adapt Faluzure (http://www.canonfire.com/wiki/index.php?title=Faluzure) a bit.

[mabriss lethe]

Giving it something like Insane Defiance could be rather fun.