During Arthas and Anub'arak's desperate flight towards Icecrown to defend the Lich King from Illidan and his army, they were force to cross through the mostly underground Nerubian kingdom of Azjol-Nerub. There they encountered still living Nerubian freedom fighters and dwarves left over from Arthas' last trip to Northrend. But as they entered deeper inside, they also encounter "horrors" (as the Dwarf leader Balegun called them) released by Illidan's recently caused earthquakes. Strange, vaugly humanoid beings that had arisen and the giant tentacles of one massive creature. As they fought their way through, they soon encountered the monster who Anub'arak called "A Forgotten One". The Crypt Lord Anub'arak seemed stunned that such a creature actually existed, saying he thought they were only a myth. He then told Arthas to "fight as you've never fought before." The undead barely managed to defeat the creature but as more faceless ones appeared and the cavern began to collapse they were forced to press on. Eventually they made it out of Azjol-Nerub and did battle with Illidan, but not until after surviving great peril.
Anub'arak's experience suggests that there is some ancient and forgotten race that was buried underground, of whom only myths reached the Nerubian people. As we know now the Nerubians are descended from the ancient Aqir people; the prospect of an even earlier race raises many questions. They only appeared in the Warcraft universe briefly in two missions (in Frozen Throne), and thus almost nothing is known about them.
Molthor in Yojamba Isle refers to Hakkar the Soulflayer as a Faceless One.