According to Freya, Cenarius, and the Tribunal of Ages, it was a tribe of dark trolls that eventually evolved into the night elves.
According to scattered reports, dark trolls have gray to black skin and lead a mostly subterranean existence. Alleged sightings of dark trolls usually involve hulking and brutish creatures who attack with little tactical planning or coordinated effort. Thus, it has been theorized that dark trolls are much less intelligent than other trolls.
There is no evidence that the dark trolls have ever formed a civilization or nation. Indeed, their estimated numbers are so few that their existence as a distinct category of troll is considered highly questionable. In consequence, it seems likely that if the dark trolls do exist, they are derived from a single troll tribe. Some have decided to classify this species as the fifth category of trolls. However, only one organization is known so far: The ShadowTooth Clan.
When Grom Hellscream first arrived in Ashenvale Forest, he found and destroyed a dark troll settlement. Later, small settlements of dark trolls were destroyed by Tyrande, Malfurion, and their army during their search for the Druids of the Talon. However, a group of dark trolls named the ShadowTooth Clan joined the night elves in the Battle of Mount Hyjal.
As of Cataclysm, the dark trolls are nowhere to be found on Hyjal. Brann Bronzebeard discovered that the Twilight's Hammer wiped them out. It's unknown if any Dark trolls remain, if some do remain it wouldn't be farfetched for the remnants to join the Zandalari.