The Darkspear Trolls are, as of the end of Mists of Pandaria, the most powerful tribe of trolls in all of Azeroth. Their leader, Vol'jin, sits atop the Horde as its new Warchief, the first non-orc ever to hold that title. In their time with the Horde, the Darkspears have weathered many challenges - the initial travails of their escape from the Sea Witch and the death of Sen'jin to the rise of Garrosh Hellscream and the reclamation of the Echo Isles, and most recently the ultimately successful Darkspear Rebellion that deposed Garrosh.
Once, the Darkspear were the smallest and least respected of all the jungle trolls - cast out of Stranglethorn Vale by the more numerous and aggressive Gurubashi, they came to inhabit the islands of the South Seas, where Thrall and his orcs encountered them. It's amazing to think about how these bedraggled, oppressed trolls managed to become so powerful a force. In part, it must be credited to Vol'jin. Following Garrosh Hellscream's attempt to assassinate the Darkspear chieftain, it was Vol'jin who ultimately united and led the Horde against the warchief.