Theory about Balhilfet. (Spoiler)

[RPGeek]

Let's see. He and Yxonumei (or whatever, I haven't played IWD for a long time) are enemies. Yxonumei is a Marilith (Tanar'ri).
That would automaticly assume that Balhilfet is a Lawful Evil Baatezu-Devil Pit Fiend, but wait! If he is a pit fiend, then how can he have been excluded, like he says he is?
After all, the Pit Fiends are vastly powerful monsters, who cannot be overcome by any other denizens of The Nine Hells. So where does this lead? Balhilfet isn't a Baatezu, but an exluded something-else. Perhaps he was an excluded underling of Yxonumei who made a pact with the Baatezu!
Or perhaps....
he isn't!
UWA HA HA!

[Brock Landers]

There are more powerful devils then pit fiends. Pit fiends are just the most powerful "generic" devil. (ie. non-unique)

[RPGeek]

D'oh! I forgot about the super-devils.

[Kayless]

Belhifet couldn't be a rogue Tanar'ri since he's of Lawful Evil alignment (use the infinity explorer to see for yourself). And Tanar'ri and Baatezu never work together (too many millennia of bad blood). I would stick with the most obvious answer; That Belhifet is indeed a Baatezu and that he and Yxunomei were exiled for letting their personal rivalry interfere with the big picture (the Blood War). It's not uncommon for demons and devils to oust or off each other at the first opportunity.

[RPGeek]

Balhilfet may be Lawful Evil, but that doesn't mean he's always been that. That's what I meant...well, hmm...I just read what you said. Dammit. Nevermind.
All this just because I forgot about the super-devils. Well, at least I know their names. Uwahaha! Well, wait. Dangit. They changed the site. Damn, it's all lost.