Neltharion had not appeared here to help Illidan and the others. He had come for the Dragon Soul.
Although he had previously escaped the battlefield with the Dragon Soul, its influence on Neltharion gradually began to show after he returned to his lair. The completion of the Dragon Soul had, in truth, marked Neltharion's complete fall. Evil power began to breed within his body, not only changing his original appearance but also further eroding his sanity, until madness gained the upper hand in his mind.
The immense changes to his body brought immense pain, forcing Neltharion to ease it through slumber. During that time, he also ordered the goblin craftsmen in his service to forge him a suit of armor to hold together his body, which had begun to crack apart.
Yet what Neltharion had not expected was that before the goblins could finish forging the armor, someone had stolen his Dragon Soul while he slept!
He did not know who had taken it. He only knew that he had to get the Dragon Soul back!
Thus, after being jolted awake, Neltharion followed his connection to the Dragon Soul all the way here. But by then, Mannoroth had placed the Dragon Soul above the Well of Eternity, and the barrier had already formed.
How could Neltharion care about any of that? Through the barrier, he saw his precious artifact right before him, and with a single-minded determination, he tried to break through the barrier, rush over, and reclaim what belonged to him.
Naturally, his rampage helped Illidan and the others. The Black Dragon King Neltharion's power was overwhelmingly great. Under his furious tearing, the barrier began to flicker and tremble on the verge of collapse. Seeing this, Illidan and the others, who had already begun to despair, suddenly found hope