Anything to be stronger

[Sydonai]

"I traded away my human weakness for power long ago."
While the game might not have been that good this quote is perfect for the character I'm making.

An antagonist that wants to be the most powerful being in existance and is willing to go to any lengths to attain their goal.
How would this character go about attaining power, I mean in-character power, not purely mechanical power based on loopholes or odd readings of RAW.

I don't even know the characters race yet, but I'm leaning toward Hellbred; He/She was almost damned to the Pit but was given another chance, and decides to never allow The Powers That Be to have that kind of control over their existance ever again.
Of course, this backstory is mutable, nothing is set in stone yet.

[Amnestic]

Given your description, I think an Ur-Priest would fit quite well thematically.

[The Rabbler]

You could always just go kobold and actually have a lot of power.

[Tvtyrant]

Ur-Priest MT sounds about right with the goal of double 9s.

[faceroll]

You can use planar binding to bind fiends. I recommend some of the fiendish symbionts in the back of the fiend folio.

Grafts and/or lichdom also seem like the way to go. Or, if you want mechanical power, necropolitan and bought off evolved template.

You could go incarnate warforged necropolitan with the evolved template applied 3 or 4 times, and bought off repeatedly. Make sure you get someone to perform the ritual on a desecrated altar and has corpsecrafter feats.

Spells-to-power Erudite 3/wizard1/Cerebremancer10 gets you mastery of both the arcane and psionic arts. Well, not really mastery. More like mastery of the psionic stuff and decent wizard casting until level 14.

[Sydonai]

An Ur-Priest Lich? Sounds about right.
What about Elder Evils, I can see this character allying with Atropus. She would view it as a "partnership" more than anything else, she won't ever worship or serve anything ever again, but she is more than willing to become the ally of an unbeleivably strong being, especially since Atropus' goal is something she would work towards anyway. But would that be enough to gain the bonus feat? She wouldn't do it if there wasn't anything in it for her.

edit: This is an NPC BBEG, not my PC.
edit2: This is sounding to much like the NPC in Atropus' chapter of Elder Evils, any ideas on making her a little more unique.

[faceroll]

If it's an NPC, I would definitely give it dual casting. Maybe psionics + ur priest? Full plate lich:
http://images.wikia.com/lotr/images/5/59/Witch-king.jpg

[Runestar]

IMO, the best thematic fit would be classes/prcs which have a pact-based theme, such as warlock, binder or pact-based adept. In game terms, they would have many class lvs, but you could fluff it as them receiving them rapidly over a short period of time in exchange for some terrible price.

I am thinking warlock/hellfire warlock. Specifically, the fluff for Davoren found here.

http://www.wizards.com/default.asp?x=dnd/frcc/20070314