Okay, here's my hypothetical history of the Tain and related creatures.
The Myth world connects onto another world/dimension/planet where the local gravity is considerably weaker and time moves much more quickly. These two may be related; perhaps the gravity is felt to be weaker because temporally accelerated muscles can exert more force/power against it. But that's just a guess, unlike the rest of this carefully researched monograph.;)
Some time after organic life was first created on the Myth world (post-Trow, that is), a portal was opened between it and the other world long enough for a few Earth lifeforms to enter it, which included fungi and spiders. As centuries passed in the Myth world, hundreds of millions of years passed in the other world and the Earth-descended creatures were able to develop into a global ecosystem. Without gravity to worry about, the spiders were able to grow to immense size and, as their brains grew, they evolved human or superhuman intelligence and surpassed the races of the Myth world both technologically and magically.
Eventually the spiders contacted the Myth world and encountered a population of dwarves. Using their advanced magical/technological abilities (and the fact that they were really big) to convince the dwarves that they were gods, the spiders engineered a secret dwarven cult. Drawing on the superior knowledge of the spiders, these dwarves were able to create technomagical wonders and became known as the Smiths of Muirthemne. Meanwhile, their assistance on the Mythworld end allowed the spiders to strengthen the link between the two worlds.
However, the spiders were still unable to physically come to our world; transporting their great bodies cost too much energy and they couldn't survive in our gravity. Accordingly they engineered smaller, less intelligent versions of themselves to make the jump, a magical feat powered by a sacrifice on the Dwarven end. These smaller spiders began covertly to colonize the depths of the Myth world.
In time, the existence of the spider-cult became known and the Smiths feared Connacht's wrath. With the aid of the otherworld spiders they constructed the Tain, where they might safely entertain visiting spiders, store and make sacrifices, and do everything the general Cath Bruig population might disapprove of; in addition, the Tain was midway between the two worlds and so creatures could pass more easily between the Tain and the other world than they could between the Myth world and the other world. The Tain partially shared the accelerated time of the other world, and thousands or millions of years would pass inside between each visit by the Smiths. During this time the smaller spiders colonized the Tain, as did many other species from the other world, adapting to their new environment.
Although the cult's focus of activity had moved from the Muirthemne catacombs to the Tain, the continued disappearances and whispered rumors over the next few years were not ignored by Connacht, and the Smiths, fearing extermination, felt it was necessary to placate him with a gift. They lent him the Tain, allowing him to imprison the Myrkridia within. They then engineered its theft by barbarians, ensuring that no one who knew how to use it would possess it. In spite of the gift of the Tain, Connacht still regarded the Smiths with suspicion and eventually raided their shrines. The Smiths had no recourse but to flee to the Tain, with which they had maintained a magical link after giving it to Connacht.
Upon entering the Tain, they found their spider masters highly displeased; the Myrkridia transported in by Connacht had devastated the colonies of smaller spiders. The vengeful otherworld spiders withdrew all support from the Smiths and commanded their smaller brethren to ignore them or treat them as enemies. The Smiths constructed lightning towers and other defenses to defend against the Myrkridia, but to no avail; after the dust had settled, the Smiths and all animal life in the Tain had been wiped out by the Myrkridia (save for the smaller spiders, which could always escape to the other world.) Starving, the Myrkridia turned on each other (we know this from the fact that the skull platforms seem to contain Myrkridia skulls) and in the end were almost exterminated, though a small population of them survived to be discovered by Soulblighter after Balor's defeat. The smaller spiders continued to range throughout the Tain, but owing to the lack of resources they never lived there permanently, instead briefly visiting from the other world.
How's that for baseless speculation?
--SiliconDream
