Apologies for being a day late on this. I just finished up finals for my summer session and it has been a little chaotic, to say the least.
So, if you've ever played Horde (because I doubt you were paying that close of attention during For The Alliance), you'll remember that the hall in which Thrall resides is home to a number of other npcs as well, including a troll boss named Vol'Jin.
So who is this Vol'Jin, and why is he a boss?
A long time ago, all of the Darkspear trolls lived on some islands in Stranglethorn Vale, under the rule of Vol'Jin's father, the famous witch doctor Sen'Jin. The Darkspear were constantly under attack by humans and murlocs, and were nearly wiped out if not but for the aid of Thrall and his orcs.
Vol'Jin became chief of the Darkspear after his father died in Stranglethorn defending the tribe, and stayed behind with some of the trolls while the rest of the tribe fled to Kalimdor. The group in Kalimdor fought alongside the orcs against the Burning Legion.
When the war had ended, Vol'Jin brought the rest of his Darkspear overseas to Kalimdor, settling on the Echo Isles off the coast of Durotar, but it wasn't long before the humans were after them again, this time in the form of a fleet commanded by Grand Admiral Daelin Proudmore.
According to WoW-Wiki:
Vol'jin aided Rexxar, Rokhan, and Chen Stormstout in evacuating the Darkspear tribe, turning the three into wyverns allowing them to attack the Alliance fleet from above, and supplying them with a squadron of bat riders.
Unfortunately, they were outnumbered, and Vol'Jin fled with his tribe until the Tauren and ogres joined in the fight. The humans were defeated, but even then the Darkspear were not free from torment.
One of their own, a witch doctor named Zalazane (which many Hordies remember from their noobie quests!) began using his voodoo to enslave the trolls, and Vol'Jin and his remaining people fell back to the mainland of Durotar, living in what is now Sen'Jin village (named for Vol'Jin's father).
Vol'Jin can be found in Thrall's chamber, where he advises the warchief on various matters, and watches over his small tribe of trolls.