Faceless ones are a mysterious race of beings who serve Old gods.
Many years ago the faceless ones fought endless wars against each other old god. The three old gods in the war were N'Zoth, C'Thun and Yogg-Saron. Since those wars, the old gods have been imprisoned and centuries have past, but to this day the faceless ones still serve their masters. The numbers of faceless ones are almost endless in power beyond reckoning that the old gods have.
Arthas's Second Visit to NorthrendEdit
During the desperate flight of Arthas Menethil and crypt lord Anub'Arak, to defend the Lich King and Illidan Stormrage, the Scourge forces were forced to travel underground. The underground region jusdt happened to be nerubian kingdom Azjol-Nerub. At first, the invading undead army preyed only on the leftovers of still living nerubian resistance fighters and dwarves that were left over from Arthas' last journey to Northrend. As Arthas and Anub'Arak ventured deeper into Azjol-Nerub, they found "horrid creatures", this is the way Baelgun Flamebeard described them. He added that they came from the dark Northrend depths due to many earthquakes over time.
When first spotting the faceless ones, they were accompanied by tentacle creatures known simply as forgotten ones. Anub'Arak seemed familiar when seeing the faceless ones. The Lich King and Scourge struggled very much when trying to defeat the forgotten one, but they were successful. Right after that more faceless ones came flooding into the area and the dark depths began collapsing. This forced the Lich King and the Scourge to flee. Eventually Arthas escaped and confronted Illidan Stormrage, Illidan's worst peril yet.
Resurrection of Cho'gallEdit
At some point in history, faceless ones began attacking Theramore, a human stronghold. These faceless ones awakened because of Cho'gall to take Med'an with him. During the battle, Med'an wove together arcane and shamanic magic to defeat the faceless ones.
Lich King's WrathEdit
Three individuals state that the nerubians battled the Scourge long ago. They dug deeper underground as they were losing when fighting the Scourge. One stated that when they dug deeper, they found tendrils under the Old gods' influence, that were faceless manifests. The nerubians then ended up losing the war since they were fighting two separate forces. This misfortune caused the War of the Spiders and ultimately Azjol-Nerub. All of this points to the fact that the nerubians discovered the faceless ones and forgotten ones. The one nerubian who gave all of this information was Kilix the Unraveler.
Not only were faceless ones found under the central part of Northrend, but some were strangely found under Icecrown in a place called Naz'Anak. Down here adventurers discovered that faceless ones have battle cries with Yogg-Saron mentinoed in them which makes these dark fears about the faceless ones true.