I recently acquired Pathfinder's rockin' new Glabrezu mini (titled "Treachery Demon"), and it makes me want to use it.
For in-game reasons, he must have 32 Hit Dice, but I don't want him to have any more than 18 Racial HD, because I want him to stay in the Huge size category.
I'd like his CR to be somewhere above 20, but I'm not sure how far to go.
I started thinking of good classes to put on him, and Fiend of Blasphemy from the Fiend Folio came to mind. It has one really cool ability to imbue its own SLA's onto its followers. This will be quite advantageous, as he won't have use for some of them (he's trapped in a Forbiddance effect by a Binding: Hedged Prison), so he can bestow the use of Greater Teleport on some of his troll minions, for example.
Fiend of Blasphemy also gains spellcasting, much like cleric, but with a smaller list. It gets up to 5th level spells over 6 levels of the PrC (5th by 5th, but an extra 5th level spell at 6th), as well as two domains.
My immediate thought was "darn, too bad it doesn't keep progressing in its spellcasting." This immediately made me think of Ur-Priest. So, two levels of FoB for the imbuing of SLA's, then 10 levels of Ur-Priest. This is shaping up nicely.
Then I had a wonky random thought of giving him Nar Demonbinder levels. Not sure why, I just did. The feat prerequisites of Ur-Priest get me half-way there, but at the same time, half the spells a Demonbinder gets would be a waste, being trapped inside a Forbiddance effect. So, I'll probably drop that idea.
I suppose I could drop three levels into Wizard, then advance that and Ur-Priest via Mystic Theurge. Anyway, here were some initial scribblings about CR and HD. This does make use of Non-Associated class levels, with his first twelve spellcasting class levels counting as only +6 to CR:
Glabrezu, 12 HD, CR 13
Advanced Glabrezu, 18 HD, CR 16
Glabrezu Fiend of Blasphemy 2/Ur-Priest 10, 24 HD, CR 19
Advanced Glabrezu FoB 2/Ur-Priest 10, 30 HD, CR 22
Advanced Glabrezu FoB 4/Ur-Priest 10, 32 HD, CR 24
Glabrezu Fiend of Blasphemy 2/Ur-Priest 10/Nar Demonbinder 7, 31 HD, CR 26
Glabrezu Fiend of Blasphemy 4/Ur-Priest 10/Nar Demonbinder 7, 33 HD, CR 28
Advanced Glabrezu FoB 2/Ur-Priest 10/Nar Demonbinder 7, 37 HD, CR 29
Advanced Glabrezu FoB 4/Ur-Priest 10/Nar Demonbinder 7. 39 HD, CR 31
Advanced Glabrezu FoB 2/Ur-Priest 2/Wizard 3/Mystic Theurge 8, 33 HD, CR 25
Two big questions I have at this point:
1) Would a creature such as this qualify for epic feats (for Improved Spell Capacity for Ur-Priest - I don't utilize "epic spellcasting")? I know True Dragons of Old age or older can take Epic feats, but I'm pretty sure the only others who qualify are those with 21 levels in character classes, though I could be completely wrong on this. Does my Demon 18/Class levels 15 monster qualify for epic feats for HD 21 and up?
2) Is the advancement of Spell Resistance actually codified as a generic rule anywhere? Specific monsters that are listed as possible player races (Tanarukk, Drow, Hound Archons) specify that their SR is equal to "X+class levels," but does that apply to other creatures such as a Glabrezu? If so, does it only apply for its "class levels," or would it apply to its advanced racial HD as well?
Lastly, any suggestions for other ways to make this 32 HD locked-up demon mastermind?
NB -
Though locked up, he would have lots of troll minions (and fiendish and half-fiend trolls, and Half-troll Draegloth), and aside from the Half-Fiends, the others would be his "mortal followers" who could benefit from his imbued SLA's.
The Binding (and Forbiddance) effect imprisoning him was put in place by Hathran Circle Magic, and thus is effectively not dispellable. I suppose there is a bit of a logic breach, in that the Ur-Priest Casting should allow him to escape via Antimagic Field, but it can be reined in by the fact that he is Huge, and the antimagic field doesn't encompass the entirety of his body. The danger of escape would still exist, were he able to take Improved Spell Capacity two times (gaining 11th level spell slots), and making use of the Widen Spell metamagic feat. That will probably be how he comes to the attention of the epic PC's (currently only 13th level), he will finally gain all the feats needed to free himself.
Or maybe I should stay away from Ur-Priest, and just go Fiend of Blasphemy and Nar Demonbinder for these very reasons...