Zul'jin is the leader of the Amani tribe and lead them even prior to the Second War.
History
Zul'jin was a fiece forest troll who managed to become leader of his tribe due to his great leadership. He managed to unite all the troll tribes in Zul'Aman under his command and lead raids on Quel'Thalas which no troll has been able to do and infamous for these actions. However, his forces were defeated by the high elves. There rumors that Zul'jin attempted to unite all trolls though it was halted during the events of the Second War.
Second War
Zul'jin was approached by Ogrim Doomhammer during the Second War for the forest trolls to join the Horde. He at first refuses but realized Quel'Thalas allied with the human regions and what they have done to the orcs. He decided to join the Horde when they rescued him for the humans who captured him.
Zul'jin fought with the Horde at the time of Second War but grew angry at Doomhammer for not conquering Quel'Thalas and abandoning the siege for the assault on Lordaeron. Zul'jin and his army of forest troll stayed and killed hundreds of elves. The Horde lost the war and Zul'jin and his army retreated to the forest and went back to their savage ways. Later, Zul'jin mysterious disappeared after the war and many speculated he was killed by Windrunner. The truth is that Zul'jin was tormented and captured for hours and lost his right eye. His torturers were attacked by a small troll group. Zul'jin managed to escape by cutting off his own arm with a spear.
Third War Aftermath
A forest troll tribe, the Revantusk which was under Zul'jin' leadership joined the Horde again. However, he was missing during the time and the Revantusk had to defend themselves from invaders. In reality, Zul'jin returned to Zul'Aman to rebuild his army. After Arthas and the Scourge diminish Quel'Thalas, Zul'jin saw a great opportunity to attack the blood elves and realized they're with the Horde. He considered this the final betrayal of the Horde. A witch doctor said they can get more power through dark ways which Zul'jin accepts in his anger.
Burning Crusade
Zul'jin set his plan into motion, he led invasions throughout Quel'Thalas.