Okay then, from the beginning, and with campaign / video game spoilers.
My campaign follows the plot of the old but awesome computer game, with some modifications and additions for extra flavour, and to make the plot compatible with pen and paper.
The core of the campaign is the protagonist discovering that she is one of the bhaalspawn - hundreds of children that Bhaal sired just before the Time of Troubles, hoping to prevent his death he fortold, by setting them up to fuel his resurrection.
Didn't go to plan. Lots of kids didn't die as they were supposed to, the high priestess betrayed Bhaal and tried to steal the gathered magic for herself... long story short, there are just a couple bhaalspawn left alive, and the ones who didn't die leveled up enough that one of them might actually ascend as the next god of murder - and since most of them are evil, the story has become an all out war, whole armies brought to bear and annihilated as each is trying to remove the competition.
Last time, our protagonist defeated Abazighal, a half-dragon bhaalspawn with the power to command dragons. (she didn't actually kill him - a chromatic doom from her teammate accidentally turned him to stone and dumped him into astral. But his dragon flight is broken, and he's pretty much out of the game)
Now, the next bhaalspawn to fight is Sendai, a drow priestess (ur priest?) who took over a city in the Underdark, and filled it with an army of drow and monsters. Now she's using the thousands of slaves she has access to for demon summoning and exp-draining rituals, sending abyssal assassins after the player, and looking for various ways to locate and kill her.
And the player doesn't feel like trying to scry and fry into a literal fortress full of monsters, so it looks like a siege is in order.
Now, none of the player's allies on the surface has an army they could lead a mile underground. But, an act of open heresy Sendai committed has ruffled some feathers among the surrounding drow cities. Since the player's party (badly oversized, and full of colorful NPCs) includes Viconia, a drow priestess of Shar, she took a part of the party to Underdark, and helped make sure the cities would cooperate and go to war (it was the typical drow problem of "I know it is important, but if I commit too much of my forces, I'll expose myself to my rivals." So, while the protagonist followed the main plotline, Viconia helped stage a couple coups, bringing the right drow to power.
So now, A combined army of several drow cities is marching against Sendai, composed of drow, orog and goblin slaves, undead, and even summoned demons. The party plans to use polymorph/illusions to hide their being surfacers until the battle starts, then break through the city's defenses, and assault Sendai herself while the drow army handles the rest of her minions.
Now, with just days until the armies set off, (and Sendai trying time after time to kill the protagonist with magic at range) the player could run some last moment sub-quests to get more allies to her side.
The thing is, most of the drow underground neighbors wouldn't have any stake in one or the other side winning, especially not one big enough to get them involved in a battle that contains epic spellcasters on both sides.
So, my question was, who could the player turn to, and how to convince them.
Forces engaged in the battle so far:
1 Drow - already engaged on both sides.
2 Derro - engaged on Sendai's side for whatever crazy reason she managed to suggest them.
3 Illithid - engaged on Sendai's side, with some slave exchange, and high hopes of continued cooperation if she ascends. The Elder Brain could be persuaded to reconsider, if the party can get an audience (without fighting the whole illthid city - because if they do kill the brain, the Illithid stationed with Sendai will stay with her just for revenge)
4 Beholders - Sendai has a hive mother in her fortress, giving her control of a sizable beholder force. Sadly, this is her best guarded asset.
5 Demons - both sides employ them through magic. Sendai also has some yugoloth mercenaries.
6 Elementals - Sendai has managed to make a pact with Ogremoth, the prince of earth - but she's keeping it a secret so far - he's her trump card in the battle.
7 Undead - both drow armies have a couple liches in their ranks - undead fodder is bound to be used.
8 Duergar - unafilliated so far. Might go to war against the ilithid, but it won't be worth as much as the illithid quitting. Might contribute some iron golems to the siege effort. (something like "50 axemen" would be too little to affect the outcome)
9 Kuo Toa - so far unafilliated. Also a bit weak. What could they contribute, and how to phrase the message to make them join?
10 Neogi - both sides already have umber hulks, but Neogi could provide a good deal more. Problem is, they're a race of honorless, pragmatic slavers. (and offering them enough slaves to justify possible loss of that many umber hulks is something that won't stand with the good guys in the party.
11 Deep gnomes, myconid, flumph - too weak military to contribute.
12 Dragons. Sadly, the one powerful dragon the party befriended is both silver, and currently stuck home guarding a hatchling. There is an ancient deep dragon somewhere near, but it's a vain, greedy and sadistic creature thousands of years old. How to make it listen to an offer, and what would be a good offer?
13 Assorted monsters. Perhaps a tribe of fomorians or something? Would they be stupid enough to go to an epic battle for a promise of loot? Sendai has a megalopede (controlled by a magical rod rammed into its head) - unless the party recruits a dragon, they will have to handle it themselves during the battle.
14 did I miss anyone?