Forgotten one

Forgotten ones are mysterious entities related to the Old Gods. One appeared in a mission in Warcraft III: The Frozen Throne, where it was defeated by Arthas and Anub'arak as they journeyed through the lower reaches of Azjol-Nerub. The quest to destroy it described it as "an unfathomable horror of a long forgotten time".

Azjol-Nerub
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. In Warcraft III: The Frozen Throne, there was a chapter entitled The Forgotten Ones where Arthas and Anub'arak fought through an army of faceless ones and tentacles, with a massive Forgotten One as the boss at the end of the mission. The forgotten one lead the mysterious faceless ones, and it could "spawn" armies of tentacles and faceless ones to aid itself.

it is possible that there was more then one forgotten one. The chapter being entitled "The Forgotten Ones" implies that there were more than one of these creatures, and Anub'arak points out that the forgotten one fought by Arthas was but one of the creatures by saying it was "a forgotten one".

In Ahn'kahet: The Old Kingdom, faceless ones called Forgotten Ones can be seen. What this means for the race/term is unknown.

Chris Metzen in BlizzCast 12 stated that Blizzard "had old gods in Northrend in Frozen Throne". It is unknown what he was talking about, but it may have been the Forgotten One.

Other appearances
The "skeleton" of the creature at the Master's Glaive in Darkshore is said to be a "dead" forgotten one in an "adventure hook". It is later revealed that the dead forgotten one is the faceless one known as Soggoth the Slitherer, a powerful servant of the Old Gods.

Quotes related to forgotten ones

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

Warcraft III statistics
Forgotten One
 * Level 15
 * 4000 HP (8000 HP on Hard difficulty)
 * 1000 MP (1500 MP on Hard difficulty)
 * 42-70 chaos damage (600 range, slow speed)
 * 4 heavy armor
 * Abilities: Flame Strike, Flame Wave, Spawn Tentacle

Tentacles
 * Level 1
 * 200 HP
 * 26-30 normal damage (180 range, average speed)
 * 0 heavy armor