Themdaruhl is some sort of demon, devil, or the like, accidentally released by dwarves delving for gems around 850 eighth age, underneath Zoublae. Not knowing the creature's common name at the time, they named it Mor Boldir. It was trapped in a pocket dimension of some sort, and they cut a hole into it. A company of powerful warriors and priests fought it, holding it back long enough for a group of dwarven priests to seal it into its prison, with all of them but the young dwarf who would eventually be king of Hurin, Nain Steelhand, dying in the effort. The fighters were on average about level 15. The creature is a powerful shapeshifter, among other things, and seems to be able to modify its body and powers to defeat the defenses of its foes.
It cannot teleport or otherwise escape through the material that makes up the pocket plane of the ground around the entrance of the prison. The prison is a 300' deep pit, with a heavy magical metal and stone cap on it. At the bottom of the pit is a 600' long tunnel, which winds and forks, rises and falls, following that was a gem seam. At the end of the tunnel, it opens into a small pocket plane, but the dwarves never saw into it, so they don't know its nature.
In January of 1167, the players found the cap of the imprisonment, and discovered it was very weak - possibly even broken. They divined the story, went to Hurin, and returned with a cadre of priests. Together, they sealed and buried the cap to the imprisonment, which should hold for a few more centuries.