Story plot

It could also be somethign of religious or otherwise signifigcance to the illithid race. The Githyanki have been keeping it from the illithids for reasons such as keeping it out of the mind flayer's hands so they can't use it and/or just to tick them off.

Add in that such an artifact is bound to have a lot of lingering psioinic traces, strong emotions associated with it, or other such things - and our McGuffin could be an astral storm in a bottle.

If you want to get *really* mean, the thing is just as mysterious to the illithids and to the githyanki as to anyone else. It truly belongs to Anubis, ex-power and Guardian of the Dead Gods, and he would like it back before the balance of reality falls apart, thank you very much.

Or alternatively, tie it to the bebilith. They're astral 'natives' as much as anything else could be, and have a spider like form and nests on the Astral... and some rumors hold that they are the collective manifestation of the guilt and repressed remore of the Abyss. They hunt tanarri, and tanarri are scared of them and won't hunt back. Have the PC's be carrying around something the bebilith call: "The Mother's Egg" I'm sure you can think of ways to mess with the players from there, please involve Pale Night - she needs to get out of the house some.

Or they can be cursed into not giving it away - or if it does establish a mental bond - it can start causing severe pain if taken away from them. (Like a familar bond strained and then tormented?) That does sort of assume a certain minimal level of intellegence on the part of the object.

Let's make it a random plane shifting device. Whenever it is handled with panic or alarm, it randomly teleports the user and others in a close area. The device is seemingly completely unaffected by any planar blocking as it may even teleport in or out of sigil (on occasion). Also the device does not teleport the user to a safer place, just a different place. They may find themselves in abyss while fleeing from Tanar'ri in Sigil. This is probably why Githyanki haven't used it. Now, since the users are constantly being chased, they are mostly scared to death and that makes them shift planes pretty often. And While they are jumping from baator to beastlands to (here's plenty of planar sightseeing for you!) Carceri to Elemental Plane of Air, they find themselves drawing more and more attention from fiend and angel alike. Finally PCs find themselves in Sigil, surronded by company of Baatezu, at least two times their number Tanar'ri, several archons and some one hundred innocent people(consisted mostly of old man, woman and children), and their teleportaiton artifact not working(Perhaps broken, perhaps out of energy, who knows? Perhaps this was the intention of the device all along; all artifacts are supposed to come with a curse.) Add a few faction members, a group of inevitables from Mechanus(these people are creating a too much chaos you know, appearing out of no where and all), and a number of Illithids just to spice it up. Here is an epic showdown for you; but don't worry, it will become a "struggle of survival in epic proportions" once The Lady makes her final appearence. She always does, and when she does, somebody is sure to be flayed.