Sylvanas' Forces were a group of Undead under the command of Sylvanas Windrunner.
After The Fall of Silvermoon, Prince Arthas Menethil transformed the High Elf Ranger-General into one of the Undead. Sylvanas was horrified and hated Arthas, but was forced to serve the Lich King.
Months after the Burning Legion's defeat at Mount Hyjal, Arthas, now King, had returned to Lordaeron to purge the remnants of the Humans who lived there. Sylvanas led her own force of Banshees in the southeast to prevent the humans from escaping. Later, she and her force had assisted in destroying the other villages and defeated the Paladin, Dagren the Orcslayer.
After the purge was completed, Arthas begun to suffer tremors and started losing his power. This was because the Lich King had begun losing power himself. Another result is that he begun losing his control over other undead, such as Sylvanas.
The Undead elf had a secret meeting with the Dreadlords Balnazzar, Detheroc and Varimathras. The three were plotting to exploit the weakness and overthrow Arthas. Sylvanas agreed to assist them, but in her own way.
After Arthas had escaped from Lordaeron, Sylvanas sent her banshees to lure Arthas into a trap. After successfully immobilizing him with an arrow, Sylvanas plotted to destroy Arthas, but was stopped by the timely intervention of Kel'Thuzad. She escaped and vowed to continue hunting Arthas.
Later, she was visited by Varimathras, who offered her a chance to join the Burning Legion, but Sylvanas refused to give up her newfound freedom. Varimathras replied by attacking Sylvanas' base. Knowing she couldn't defeat him with Banshees alone, she possessed not only Varimathras' Forces, but also many of the inhabitants of the forest, including Ogres, humans, Murlocs and Gnolls. She also possibly possessed Forest trolls, but likely didn't because she hated them even more. After defeating Varimathras, he surrendered and offered his services to Sylvanas. She agreed, but kept him on a very tight leash.
She later went to attack Detheroc's base. Detheroc had brainwashed a human warlord named Othmar Garithos. Sylvanas and her forces led a covert assault in the night and destroyed much of Detheroc's and Garithos' strongholds. After Detheroc was torn to bloody shreds, Garithos and his men were freed from the spell and formed a temporary alliance with Sylvanas. Unbeknownst to him, however, Sylvanas had no intention of giving them their lands back.
Sylvanas and Garithos led a final assault against Balnazzar's base. Sylvanas led from the front, while Garithos led from behind. The joint assault eventually destroyed Balnazzar's stronghold. Sylvanas then ordered Varimathras to kill Balnazzar as a final test of loyalty. Though hesitant, Varimathras complied. Garithos then demanded that Sylvanas leave immediately, but she ordered Varimathras to kill him as well, to which he gladly complied.
With the Capital City under her control, Sylvanas declared that her forces were no longer part of the Scourge. From then on, they would be known as the Forsaken.