Old-time Planescape DM back to his roots

Interesting situation. I agree with the above post that the situation reeks of betrayal (Tan'ari arnt exactly known for their loyalties.) and assasination attempts/ challanges will be many.

I would also reccomend celestial interfernece.  You mentioned that the character is CN, not CE. Ifgood old Graz thought that celestials were secret meedling in his affairs, then an apperantly important CN lieutenant would be ideal bait.

A  celestial organization would almost certainly try to turn the CN mortal to their side, ideally in way that they can eliminate the threat if he doesn't take to the idea. If he turns, he now has to help bring down Graz (and probably fail in the attempt, a pawn used by greater powers). If he refuses, they now have to flee back to the abyss while being attacked by celestials intent on making sure Graz never hears of their interference. And most players, while being able to rattle off the threats of fighting a balor from memory, are very inexperienced when fighting celestials. 

Either way you get to throw some truely dangerous encounters at them, and some nice roleplaying as well.

Thats both of my cents. 

Welcome, Planewalker.

I have an old campaign I've let drop in the last turn or two, having moved.  I try to keep ecoes of old PCs and NPCs into my on-line campaigns (BoG'r, Sigil:The Musical II)

I think this is a cool idea, because it sends the PCs back to Sigil as high-level envoys.  You could play them meeting several other Abyssal generals and proxies on neutral ground.  The PC might be able to enter Sigil, but has to be extra careful not to get mazed by The Lady of Pain.

My own $0.02 >> Ideas for NPCs

One NPC from my old campaign was Kurr, a proxy of Tou Mu, Godess of the North Star ( From 1e Dieties&Demigods ) and my attempt at a CE Cipher.  He was The last our Heroes saw of this nemisis, he was a corpse, walking through the gates of Feng-Tu.  I had planned on him coming back as a half-fiend vampire (he was half-fiend to begin with), but never got to.

"Kurrasi-Jiasa, Envoy of Feng-Tu" ; Description: Standing a lanky 7 feet tall, bald headed, with slanted black eyes, one covered by a metal eyepatch.  His teeth are sharp, like a shark's, but black, on dark purple gums.  When he angers, which he will do at the slightest breach of ettiquite, his good eye burns red, and his tounge lashes out of his mouth, up to a yard!  Yet he never strikes first, prefering to assess his foes, vebally and mentally, before using physical confrontation.  The Evnvoy wears imaculate red and black silks, and always with a midnight blue sash.  At his side are two heavy hammers with cold iron heads.  Vague Stats: High Level Priest/Monk Vampiric Cambian.  The epitome of Abyssal Diplomacy, representing Abyssal holdings of the Celestial Beuraucracy.  Able To supply small groups of unparalleled assassins and demonic martial-artists.

"Black Spawn of Jublex"  ; Description: Jigantic demonic black pudding with a sinister mind.  Woe to the burk who doesn't get out of the way when Back Spawn is sliding down the street.  Not that it moves very fast ... most of the time.  Of course, it can shoot half a dozen psudopods at once, and it can exude several powerful poisonous, acidic, sticky, or even halucinogenic secretions at will.  Vague Stats: Advanced Fiendish Black Pudding.  Lots of personal power (Tough target for assassination-type mission), but few allies or resouces other a deep knowledge of slimes and fungi, possibly fungal alchemy too.

"Mistress of the Sky", Proxy of Pazzuzu ; Description: Normally cloaked in illusion to make her appear as a beautiful woman. Vague Stats: In actuallity she is a half-fiend harpy/succubus sorceress.  Has access to huge flocks of vrocks and other fiendish avians.  Also controls a huge but secret organization of Kenku and a multi-planar cult of Pazzuzu worshipers.

"Big Baba" ; Description: Insanely fat night-hag crime boss.  Vague Stats: Runs Night-hag market and cross-planer smuggling.  Able to provide rare items, magic, and yugoloth mercinaries.  Might also have ties to Iggwilv.

"The Monster"  ; Big reptilian thing, like a fiendish dino or something.  Mindless destruction controled by a sly and coniving Yuan-ti Proxy of Demogorgon through magical means.  Demogorgon is a foe of Grazz't.  Shame if someone stole the magic leash and collar that allow that snake to keep The Monster under control.

"The Balif" ; Vague Stas: Balor, representative from Woeful Escarand with access to hordes and hordes of lesser Tanar'ri.

"The Captain" ; Doomguard leader with access to a ship of chaos, to break up the Baatezu formations.  Must be properly motivated to attent to the battle in YOUR sector of the coming battle.

"The Minions of Thanatos" ; Proxies of Orcus, one of Grazz't's most powerful foes.  Make a bunch of these as foes for the PCs, but choose a memorable one as the head nemisis.  Here are 3. ;  "The Eye of Orcus" Vamprie-Priest with magic piercings and tatoos.  Commands many undead, sent to Sigil by Orcus to limit his influence on Thanatos. {E. of O. was a villain from one of my group's adventures, to a prime world known simply as "Deadworld", a world ruled by vampires and necromancers}  "Pale Maid"  Literally, the Factol of the Dustmen's head chambermaid at the Mortuary. Has high-level access to Faction politics and gossip, and can engineer problems for the PCs from behind the scenes.  "The Rotter" Mummy (or something else that causes disease) with a heavy Sigilan accent, because a mummy that talks with cokney accent would be fun.

Of course there are plenty of Baatezu in Sigil too.  They might try to out-bid (possibly by removing bidders) the Abyssal side when negotiating with Yugoloth mercinaries or assassins.  A Baatezu General might be in town, and the PCs might be asked to take out his second and third in command as soon before the coming battle as possible.  The PCs migh intercept a part of the Baatezus' battle plan, or a fake battle plan in an attempt to fool them.  Baatezu agents might try to sew seeds division among the Proxies of the abyssal lords.