The Corruption of Teldrassil

The reason Undercity had fallen under the control of the Dreadlord Varimathras so quickly was another crucial factor: Sylvanas, the Forsaken Dark Lady, was aboard the Banshee's Wail en route to Orgrimmar, intending to further discuss the Northern Expedition with Warchief Thrall.

On his way through the portal to Grommash Hold, Rend happened to run into her, and they proceeded together.

With Undercity, her capital, having fallen at such a critical moment, the appearance of Sylvanas, the Forsaken Dark Lady, instantly seized everyone's attention.

"I need the Horde's help to retake Undercity from those traitors." It was no easy thing for the proud and formidable Sylvanas to ask for aid of her own accord. Clearly, the rebel army led by the Dreadlord Varimathras was even stronger than expected, and Sylvanas could not handle it alone.

Thrall looked toward Vol'jin, the Darkspear chieftain crouching in the corner. The two nodded to each other before he said to Sylvanas, "The Forsaken are members of the Horde. The Horde will not stand idly by in the face of this betrayal by the Dreadlord Varimathras. We will help the Forsaken retake Undercity."

Sylvanas had led the Forsaken into the Horde only under pressure from the Stormwind Alliance; theirs had been a partnership of mutual interests. When she came to seek help this time, she had expected Thrall to exploit the situation and demand concessions from the Forsaken. Yet that did not happen. She saw sincerity in Thrall's eyes, and it stirred something within her, for she had always regarded the Horde as only a temporary refuge.

"The Darkspear Clan reinforcements bound for Northrend to join the Northern Expedition have just reached the northern part of the Eastern Kingdoms. I'll have them turn around and head for Tirisfal Glades." These reinforcements had originally been intended