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.
It's hard to really understand how the Darkspear will change now that Vol'jin is Warchief. They managed to actually stand off against Garrosh's Kor'kron and push them back from Razor Hill before bringing the battle to Orgrimmar, but it can't be pretended that they fought alone. The trolls facing against Hellscream's forces did so alongside tauren loyal to Baine Bloodhoof, Sylvanas' forsaken, Lor'themar Theron's blood elves, goblins who found that Garrosh tended to ignore contracts, and even some orcs who felt that Garrosh had gone too far. While it was called the Darkspear Rebellion, it never would have succeeded had not the rest of the Horde races rallied around the rebel banner, and so, the Darkspear do not seem likely to repeat the racial chauvinism of the 'True Horde' era.
What is clear, however, is that Vol'jin's tribe has gained sufficient strength through the past several years to credibly balk the wishes of an orc warchief on the throne of the Horde. Between the true restoration of the Echo Isles, the defeat of the Zandalari when they schemed with the Gurubashi and the Amani, and Vol'jin's display of resolve surviving Garrosh's assassination attempt, Darkspears have never been more of a force to be reckoned with. And there are definitely those forces that will indeed reckon the Darkspear as an enemy to be destroyed. First among them is likely to be the Zandalari - once a distant tribe of scholars and intellectuals, the prophet Zul has led the Zandalari to seek conquest several times now, and it was Vol'jin who led the peoples of the Horde and the Alliance to stop them the first time. Then, when the Zandalari attempted to seize control of Kun'Lai, Vol'jin helped balk them again. In truth, what makes Vol'jin and the Darkspear so successful when other troll tribes fall by the wayside is that the Darkspear, alone among trolls, do not seek either to dominate or to recreate an ancient past.
The Darkspear are not attempting to reclaim the lost glory of the ancient troll empires that once ruled much of the surface of Azeroth, as the Zandalari seem obsessed with. They aren't isolationists - they're very much a group of trolls living in the here and now. It was through this willingness to embrace change and take chances that the first troll druids came to be, Darkspears such as Zen'tabra who listened to the word of Gonk the raptor spirit. While the Zandalari use strange, ancient powers to dominate and control, the Darkspear are not interested in returning to the past - this may in part be due to the fact that under the old order of things, the Darkspears were effectively bullied out of their own homeland in Stranglethorn. The hardships the Darkspears endured not only made them strong, they also made them a tough-minded group that sees their fellow trolls for what they are, and causes them to value their relations with those races that have proved themselves allies rather that forcing them to spend time trying to conquer the ancient enmity each troll tribe has for the others.
When Vol'jin moved against the Zandalari during the Cataclysm, he won because of that same sense of pragmatism - for Vol'jin, defeating the Zandalari was more important than currying favor, and it's a mindset that we've seen repeat itself. And keep in mind, Vol'jin was offered a seat at the table, so to speak, twice - both just after the Cataclysm and the next time while fighting Zandalarri agents back from Kun'Lai Summit and the Shado-Pan Monastery. Even when Garrosh Hellscream tried to have Vol'jin murdered, the trolls put the Horde ahead of the Zandalari.
Now, with Vol'jin as warchief, the Darkspear have the chance to put these ideas into action. Can they lead the Horde? Always energetic in their defense of the Horde proper (remember, the Darkspears actually built multiple settlements in the Zangarmarsh back in the days of the Burning Crusade) it's still remarkable to see that they've managed to rebuild so quickly following Zalazane's actions on the Echo Isles, and risen so far. Their role in Warlords of Draenor is yet to be set, but the Darkspear, who have themselves cut their own racial ties to be better members of the Horde, may be the perfect people to show the rest of the Horde how to do likewise.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.