While The Lord of the Rings can only pause briefly on the precipice of the deepest delvings, where far below "the world is gnawed by nameless things," we are privileged to be able to shine a light into those dark recesses, and reveal what new horrors may be found.

Hence the Rogmul: a powerful creature trapped between this world and the realm of the Unseen, enslaved to a great and ancient power of Middle-earth.

There's also insight into its naming in Chris Foster's Developer Diary, "Riddles in the Dark."