The 'Shliss' outpost was founded during the height of the Illithid Empire over ten thousand years ago, before the time of Tain McLir and during the time of the great races. It was founded by an illithid Empire colonial fleet. The Fleet made landfall on Talamh and built a colonial city. It was from here that the illithids spread throughout the underdark of Talamh and throughout the planes in the Peering Eye world.
The outpost was named Shliss, which means 'Focus', in the Ancient Illithid Tongue. Shliss was initially very successful and many Nautiloids arrived at the great dock of Shliss to deliver and trade in slaves and other valuables.
Several other colonies were founded in various other parts of the underdark and this process led to inevitable clashes with other races beneath the surface. These wars raged for hundreds of years as the illithid struggled for dominance of the underdark. Once the underdark was theirs they intended to take the surface world and claim the entire planet for the Illithid Empire.
They may have succeeded on Talamh as they had on countless other worlds but for two major obstacles.
First they paid no heed to the Gods, Fallen, Chosen or Risen and so had no divine allies, secondly the Gith revolution overthrew the Illithid Empire and so they had no off-world allies to send reinforcements. These two major setbacks meant that they were unable to conquer the underdark and that they were unable to match the power of the great races.
Needless to say the Illithid leaders persevered and hoped to achieve their goals nonetheless. The Illithid of Talamh were one of the few who managed to prevent the Gith uprising, by slaughtering thousands of Gith slaves and then sealing their city against extraplanar travel and communication. Their massive colonisation and trading fleets were at their greatest size and were used with tactical brilliance against the Gith Navy in great battles in Eyespace. The losses were horrendous on both sides and so they both sought easier targets, vowing destroy their foe once their strength was increased sufficiently.
The colonisation had stalled considerably due to the losses in the Gith Wars and so the Illithids began a long period of consolidation, growing their strength, in preparation for their eventual conquest.
They declined to worship any gods though. The Risen were anathema to their beliefs, the chosen required them to renounce their dreams of planar conquest. And while The Fallen suited their purposes and outlook, they were too few in number and worshiping them provoked to much antagonism from both the chosen and the risen. The Illithids didn't feel that is was necessary to share power with anyone.
Hundreds of years after the fall of the Metarex, Shliss was one of the few major Illithid centers still active. A ragged flotilla of Nautiloids arrived from outside the sphere. They bore the emblems of the Metarex imperial family. They came seeking shelter in Shliss and declared that they had brought a weapon of great power in order to allow the forces of Shliss to take over the planet. Talamh would then become the staging point for the recovery of the Empire. Negotiations took place for the positions of the newcomers in the Shliss government and for Shliss role within the future Empire. The newcomers who were members of the imperial family did not get the respect they felt they deserved, while the Shliss rulers did not believe that the Empire would ever be reborn and so were disinclined to give up their powers to the haughty self-important refugees.
A nasty battle erupted and the imperial family was slain by their Shliss Kin. Most of the Shliss rulers and their best warriors were also slain in the viscous battle. Their ships were ransacked to find the 'weapon of great power' but it was never found. The Shliss Illithids believed that the Imperial Family was lying about the powerful item in order to gain power within Shliss. They were quickly dismissed as imposters.
Years later, disaster struck. The last Scion of the Metarex an Illithid child called Teth'Cthu, son of one of the Imperial Refugees had escaped into the underdark. He was approached by Athru, who offered to become the protector of the Metarex Illithids. But only if they would worship the Fallen and do Athru's bidding. Teth'Cthu agreed readily and quickly became one of The Fallen's Minions. Athru sent protectors to the child while he grew in power. Teth'Cthu began to recruit followers from among Imperial Loyalists and the disenfranchised illithid in Shliss and other settlements. Then he slipped into Shliss central birthing pool and with Athru's power caused a mutation in the spawn of Shliss' ruler. Teth'Cthu barely escaped with his life and was badly burned by the Mutants' acid breath.
The Mutant, a Neothilid grew in a blinding flash to a gargantuan size and proceeded to slaughter all of the inhabitants of Shliss. The rulers quickly sealed the city and the docks and called all their brethren to the birthing pool to destroy the creature. The battle raged for days, with massive losses on the Illithids side and no substantial damage to the Neothilid. Then the rulers decided to flee to the ships. The Neothilid anticipated this and ambushed them on their way to the docks. The first illithids to reach the docks were a group that were loyal to Teth'Cthu. They closed the gates to the docks against the rulers and left them to their deaths. The Neothilid massacred the rulers. However having suspected that there was treachery involved, they had trapped the imperial Nautiloid with poison. The loyalists attempted to use this nautiloid and were poisoned. Meanwhile the remainder of the Shliss population fled through the smaller unsealed air and sewer tunnels into the underdark to seek out the other illithid cities.
Teth'Cthu was waiting for them, hideous in his burned flesh and forced them to convert to The Fallen or be destroyed. Few escaped him. Descendants of Teth'Cthu still rule a vast number of Fallen Illithids but there are still many independent cities scattered throughout the underdark. Some are followers of The Chosen or of Aigne having with the Church in a bid to fend of the growing power of the Fallen.