Faceless ones (aka faceless) are mysterious monsters that reside in Azjol-Nerub and Northrend like in Icecrown. They are vaguely humanoid creatures who, as the name implies, have no discernible faces insofar as other races recognize them. One arm is much larger than the other, grotesque and misshapen.
Tales say they lived in Azjol-Nerub long before the Scourge came, dwelling too deep for the nerubians to hunt them. The strange faceless ones are ancient monsters long imprisoned beneath the ice and recently loosed again. ( Faceless ones have a long magical heritage. They draw upon their race's legacy of arcane spellcasting traditions to increase their power. Although faceless ones have a racially low intellect, they have a strong heritage of necromantic magic. ) ( A second passage into the )Old Kingdom has been discovered by the Scourge, and Baelgun knows the faceless ones will overpower even the undead and then be free to escape into Northrend proper, and so he vows to stop them. ( )
During Arthas and Anub'arak's desperate flight towards Icecrown to defend the Lich King from Illidan and his army, they were forced 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 delved deeper into the ruins, they also encountered "horrors" (as the dwarf leader Baelgun called them) released by Illidan's recently caused earthquakes: strange, vaguely 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.
In the quest The Faceless Ones, Kilix the Unraveler says that as they fought the Scourge, they dug deeper underground as they lost ground to the undead. "We inadvertently exposed a tendril of the old god's will, which was made manifest in the creatures known only as the Faceless." He then goes on to say that they lost the war because "In the end, we could not fight on two fronts against such powerful enemies. Our misfortune cost us the war against the undead, and ultimately, our home." implying that the nerubians discovered the faceless ones on accident. Interestingly, "faceless ones" and "forgotten ones" look similiar.
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 the Frozen Throne), and thus almost nothing is known about them.
The Curse of Flesh instigated by the Old Gods was intended to make the nascent races susceptible to "assimilation". It is, therefore, entirely possible that the faceless ones are the unfortunates who were successfully assimilated.
Hakkar the Soulflayer is referred to as the "faceless blood God" by Primal Torntusk in the Hinterlands and as "the faceless one" by Molthor at Yojamba Isle. One piece of loot that Hakkar drops is the Fang of the Faceless. It is not known if this description of Hakkar indicates a connection to the faceless ones that Arthas encountered or if it is simply a title.
Also, these creatures could be connected in one way or another to the Old Gods. The creature Arthas and Anub'arak destroyed bears a few physical similarities to C'thun. Hakkar is also an agent of the Old Gods, demanding blood tribute from the trolls (who were enemies of the aqir). Millenia ago, Hakkar created a schism between the troll tribes, shattering the troll empire.
Since it is never specified what the avatars of C'thun looked like, it may be possible that these faceless ones or forgotten ones are the avatars. After the split of the Azj'Aqir empire, C'thun obviously lost "control" over the Azjol-Nerub "sub-empire", therefore after the countless millennia since the split, it may be possible that the faceless ones are the avatars of C'thun or forgotten ones.
The faceless one mobs in the Old Kingdom are actually called forgotten ones, which may suggest that the term forgotten one in fact refers to a faction of some sort.
Also, Krilix who supplies quests for Azjol'Nerub speaks of the monsters as being in some way connected to the Old God:
"In my peoples' struggle against the forces of the undead, they dug deeper as they lost ground. In desperation, they dug too deep.
We inadvertently exposed a tendril of the old god's will, which was made manifest in the creatures known only as the Faceless. "
There are many ways to interpret this statement, among them that in breaking free part of the Old God, the nerubians released his slaves. One could also argue that the Old God "manifested" the creatures out of himself as creations or extensions of himself. The forgotten one in WC3 did seem to have similar powers or creation. This might explain why the faceless ones are actually called forgotten ones in WOW.
Then again, Krillix could have just been speaking metaphorically as he goes on to refer to them as separate "insane creatures" only belonging to the Old God.
In Wrath of the Lich KingThis section concerns content exclusive to Wrath of the Lich King.
Faceless ones are also encountered underneath Icecrown Citadel in a place called Naz'anak: The Forgotten Depths. Killing the Faceless Lurkers there is required for the quest  . Much like gnolls or ogres, faceless ones have a "battlecry": "Shur'nab... shur'nab... Yogg-Saron!". It would appear, therefore, that the darkest fears about the faceless ones are true.