Garran: The Goromorg had originally harnessed the power of the 'cube' to construct the dungeon, but they found that they couldn't properly control it, and thus engineered its imprisonment. However, they'd also become bound to the 'beast' (as they called it) in the process, thus the tapestries depicting them chained to a gear.
The cube retained enough power to reach out into the minds of visitors, and it became (literally) a game between the two (equally malicious) forces. Both sought release from their respective bondage, although in the case of the Goromorg, that release would also mean their demise, especially since the cube couldn't be destroyed without first repairing it, something they were unwilling or unable to do.
For its part, the cube hadn't counted on the contingency laid down by the architects in the form of the weapon of power (and the fact that only outsiders could reach it). It figured that, once it had been released, there would be nothing capable of stopping it, and the story implies that, given time, its power would rapidly have grown and made it a threat to the whole region.
Sorry for the thread necro, but I just beat the game last night.
Thanks for this explanation. You caught some things I missed, particularly that the Goromorg's fate was bound to the Undying One. Without that bit of information, I was really frustrated with why they didn't just kill the Undying One as easily as I did. Their "binding" to him also kind of explains why he's called "The Undying One" when he was, in fact, quite killable (die-able?).
One thing that disappointed me: hiding Toorum's bones in a secret area. He's a major "character" in the game (in a certain sense), and I think it's a little unfair that you can only find out what happened to him through a secret area. I missed it, but admittedly, I was playing it late at night and got caught up in trying to finish the game more than find all the secrets. Maybe it was a fairly obvious "secret" that I just rushed past.
Finally, I found the final battle to be extremely frustrating. To me, it didn't just seem to be a matter of "git gud." There were so many spawned enemies clustered together that if you got a bad teleport into the room, there was practically no way to avoid getting trapped and smashed by the Undying One. Let alone getting torn to pieces by the Ice Lizards, Ugguardians, and others. The result was that I kind of gave up on playing it "fair" and used "pit-scumming" to beat the Undying One--I'd stun him, take a piece out, and then jump down a pit, rest, and finally teleport back to the room. It was cheesy, but whatever, it worked.