From WoWWiki

The forgotten ones are massive and powerful tentacle-bearing aberrations living in the deepest reaches of Azjol-Nerub. Forgotten ones seethe below, eager to return to the world above.^{LoM 91} One appeared in a Frozen Throne mission, where it was defeated by Arthas and Anub'arak as they journeyed through the lower reaches of Azjol-Nerub (though others may appear in WotLK).

The forgotten ones lead the mysterious faceless ones. They seemed to be able to "spawn" its army of defenders. The forgotten ones can charm units, spawn tentacle wards and use flamestrike.

Possible C'Thun link

This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore!

• See Old Gods speculation.

Quotes

Anub'arak: It cannot be... A forgotten one! Look to your defenses, Death Knight! Fight as you've never fought before!