Recruitment was circumspect, and the level of secrecy is extremely high. You were brought (one way or another) to the Rock of Bral by the Illithid slaver operation, which seems well organized and deadly efficient. The pay is excellent, but that's only part of it -- people like you (and the tentacled ones, you guess) are always getting stepped on by those who think they know better than everyone else, the self-righteous morons. Well, now it's time for some payback, and you've been recruited to mete it out. Should be fun, in a grim way.
Bral is a well-known asteroid city, a mercantile hub for all of Wildspace, ruled by a Prince, Andru, whose policy is peace for prosperity, meaning anything for money. Though slavery is officially outlawed, so is much of the economy on Bral, where anything that can be shipped can be bought. The only truly serious offense on Bral is fooling with or fouling the somewhat limited atmosphere. Everyone will cooperate to hunt down a party using fire-based spells or spells like Cloudkill, and weather spells require Royal permission.
What is much less well known is that Bral was a Forge, a gigantic dwarven spelljamming mountain from ancient times, abandoned when the valuable ores ran out. Its interior is riddled with tunnels and caverns, mostly beautifully hand-hewn, for the dwarves' mining and manufacturing base. The oldest dwarven sections, closest to the surface and near the modern-day spelljammer docks, were vast underground warehouse complexes, and this is where the Illithids base their operation. The old open-bar cages for goods are ideal slave pens. The underground stream, cycling endlessly to and from the surface lake due to the effects of the Rock's uneven gravity plane, supplies fresh water.
The caverns go on for unmapped miles. Most are unexplored, and the curious don't return. Efforts to scry the underground corridors themselves have been mostly unsuccessful, as have divinations to find out why. The Illithids are interested only in their defensible area, and consider others' enforced ignorance to be an advantage.
The Royal family has raided the slavers from time to time, when their political situation would be strengthened by a show of legal force. Lately, adventuring bands impatient for results (or perhaps taking the slaves for themselves) have been doing the job for them, thus saving Royal expense, so the Prince has closed his eyes to the brief disturbance of the peace that sometimes ensues. After all, the Illithid ambassador can't protest an illegal raid on an illegal operation.
The Illithids received information from somewhere that there was about to be yet another raid. They've increased the security, trying to make their underground fortress impregnable. Perhaps it is, and perhaps it isn't, but that's why you're here: you and the rest of your high-powered force are going to try.
Over the past week or so, you and your companions arrive from distant ports, worlds, and planes, by spelljammer ship, magic, or other means. As the days have worn on and your forces have assembled, you've heard some of them voice the complaint that things are taking longer than expected. Upon arrival, you were escorted under cover directly to your quarters. The only others you've seen (besides your fellow recruits) are those who seem to work here, and they look like they wouldn't be given to conversation even if it weren't for the atmosphere.
Under orders, no one has visited the city of Bral or even seen outside the compound; security has been tight for obvious reasons. The edgy mood got worse when the biggest and meanest of your fellow recruits, an enormous half-minotaur ur-priest-assassin with a murderous glare and a blood-drinking axe (who called himself "Brute"), decided he would ignore the orders to stay in the Compound. A couple days ago, after you'd all had time to get familiar with most of the caverns in use and find your way around... late at night, Brute decided he would go drinking topside anyway, and slipped out somehow.
The next time you saw him was when Arkesh, the Force Leader, brought him back... in pieces. You never saw what happened to the head, but you had no trouble imagining it anyway.
Everyone is concerned about getting more information, what little bits might slip. Unfortunately, while the food and quarters have been quite adequate, the information hasn't. You've been provided with maps, and a chance to look things over. You've met some of your fellow recruits. Some have been talkative, some not. You've met Arkesh, who seems to be organizing the action, and has treated you and the others evenhandedly, but hasn't had more information to offer either. No one is quite sure what's going on in Arkesh's mind, or how exactly such a strange creature came to be -- an Illithid seemingly sewn together from pieces of other various Illithids, but unlike a golem or an undead, very much alive.
Arkesh has summoned all of you to a meeting with each other in the Living Quarters, the first where everyone is in attendance; apparently the time has arrived for you all to begin planning in earnest about the coming attack. There is a lot of thinking to do, about possible traps and ambushes, magical and non-magical defenses and counterattacks, what to do if slaves are caught in the crossfire... The slaves are what this little game is about, after all, and you know that neither side wants to have them dead.
Your intructions are simple, though your task is not: plan as best you can, for time grows short. Coordinate your efforts well; the enemy's strength may be uncertain, but is certainly too strong for any disorganized force to have a likelihood of success. Feel free to ask questions, and gather whatever information you can, but there may not be much available, so be prepared for contingencies. Be prepared for the worst.
Be prepared.