Psionics was not to my liking in AD&D so it's no surprise that a couple of decades ago I overlooked this adventure. Going through it now however, I can see a place for it in a Ptolus campaign.
Originally the module is set near a small town in some forested hills where the Beholder that kicks the story off resides in the caves. The Beholder attacks the town without assaulting people, destroying only buildings and objects. The goal is to draw would-be heroes to its lair where it hopes to convince them to investigate something it discovered in the deeper caverns below.
Opening the adventure with a Beholder attack in the city doesn't feel right to me since there would likely be quick reprisal and little chance of negotiation if things begin with violence. Of course anything is possible. It seems more likely that the Eye beast would be fully aware of how dangerous that would be and would instead choose a saner approach.
Acting through one of its minions perhaps a message is carried into The Ghostly Minstrel and the not-too-bright creature seemingly at random, drops it onto the table where the player characters are sitting. Yes, it's the "you're sitting in a bar" trope, this time the monsters are seeking help. Then the minion turns and walks out, all without a sound. Scratched into the surface of the large piece of slate is the phrase "follow it".
Assuming the players know a plot hook when it's just been shown to them, they should follow the creature through part of town, maybe near the city wall in North Market, and down into the sewers and dungeon below, eventually getting to the caves in which our quest-giver resides. Have the miniion enter ahead of the party and be one of the two victims of the giant Trapper/Lurker in area #2, At that point the adventure can run almost as written.
I suggest placing the caves and deeper caverns of this scenario under the area just outside the city walls to the north of Ptolus.
There are Derro thralls as well as Derro mutants in the adventure as written, but I vaguely recall Derro not being part of the Ptolus setting. I don't recall where I saw that information. Don't let that stop you from using them in your campaign. In this adventure it might make sense because of how different everything in the citadel is from the norm.
There is a lot of system conversion work to be done if you're running this in something other than 2nd edition AD&D.
In the great cavern below is a floating crystal and stone citadel inhabited by illithid variants that are hostile to regular mind flayers, but might not be hostile to the characters depending on how the party behaves.
The evocative descriptions of things within the crystal citadel make for a fascinating experience.
It is possible to make allies and enemies of various NPCs from the adventure with opportunity for future plots.
Lazat Lan is a prisoner that looks human, but is an illithid that for some reason did not transform fully when undergoing ceremorphosis. He will try to get help escaping, and given a chance he'll turn on the PCs, maybe even trying to crack open one's head with a weapon in order to eat their brain. The potential for shenanigans in the city exists if this NPC escapes into Ptolus.
Once their exploration of the citadel is finished and the party returns to the beholder, it will expect a full report as well as the share of treasure it demanded at the start. Anything less than it expects will trigger an attack. The party needs to be ready for that eventuality. Escaping from the beholder without vanquishing the foe will leave them with a recurring enemy.
Overall the module will probably be great fun to play through and players will get a good sense of the culture of both illithid and their abnormal variants.
