The Witherbark tribe split off from the Amani Empire after the Troll Wars and returned to their ancestral lands, in the areas that are now called the Hinterlands and the Arathi Highlands. Like all other forest trolls, the Witherbarks allied themselves with the Horde during the Second War under the leadership of Zul'jin, a famous troll hero from the Amani tribe. When the Horde was soundly defeated, Zul'jin disappeared, and the forest trolls abandoned their orcish allies.
The Witherbarks living in the Hinterlands have since happened across the Revantusks and launched many attacks against the small tribe, but the dwarves of Aerie Peak have also become a noticeable threat. In the meantime, the Witherbarks living in Arathi Highlands have grudgingly allied themselves with the Boulderfist ogres in order to wipe out a far more hated enemy: the Horde forces who have constructed the settlement of Hammerfall.
In World of WarcraftThis section concerns content exclusive to World of Warcraft.
Warlocks in the tribe
At Boulderfist Outpost where the Boulderfist ogres as well as the Witherbark tribe have a mine together, Troll Warlocks as well as Imps can be found. Those warlocks are separated from the rest of the tribe.
Speculation about the Warlocks
There is no official source how they learned to summon Demons or use Warlock abilities but the only logical ways would be that they learned them from orcish Warlocks or were contacted from Demons. The more likely way is that were not able to get back to their tribe after the Second War, so that they had to work together with Boulderfist ogres and a few Orcs, so that they could learn from orcish Warlocks and survive thanks to the new powers they gained. A third speculation is that one off the Loa this tribe worships to is a Demon of The Legion, so that they worship The Legion without knowing it. There are quests in which you have to fight for Stromgarde and kill Warlocks as well as Ogres.