Sylvanas' Forces

Sylvanas' Forces were a group of free-willed undead and soul-stolen creatures led by the Dark Lady Sylvanas Windrunner who would eventually become known as the Forsaken of Undercity.

Early story
Illidan Stormrage's remote spellcasting in Dalaran (intending to destroy Lich King's stronghold of Icecrown and using the Eye of Sargeras) failed in completely destroying the Dark Lord's hold on the frozen north but clearly damaged the Frozen Throne and resulted in Ner'zhul's power wane and leak daily, crumbling on his capacity to keep control over all of his mind-controlled undead forces, and so many of his once-mind-controlled minions took opportunity to break free from his grasp, including the Dark Lady Sylvanas Windrunner.

So, the banshee gathered a notable part of the now-free-willed undead to her side, including many of her Quel'Thalas former countrymen.

After, she had a secret meeting with the Burning Legion Dreadlords Balnazzar, Detheroc and Varimathras. There she understood why they were free; the Lich King's weakening, incapable of widely mind-controlling all of his undead minions. The dreadlords were plotting to exploit his weakness and overthrow the Dark Lord's greatest champion. Sylvanas agreed to assist them and a small alliance (of convenience) was formed.

The Dreadlord Insurgents started to plot against the self-styled king Arthas Menethil, making a trap, attempting to kill him. Arthas was able to escape from the Dreadlords' trap from Lordaeron Capital City, killing a powerful abomination in the process. After Sylvanas sent her banshees to rescue Arthas and escort him across the river but in truth, she was luring him into a trap. Ner'zhul warned Arthas that he had been deceived, but was too late as Sylvanas successfully immobilized the death knight with an arrow. Sylvanas was about to destroy Arthas but was stopped by the timely intervention of Kel'Thuzad. The Dark Lady was forced to retreat but vowed to never stop hunting Arthas.

Declaration of war
Later Sylvanas was visited by Varimathras, who offered her a chance enter the Burning Legion through joining their new order, but Sylvanas refused to give up her newfound freedom and remorsefully insulted the dreadlord stating that she lived as a slave long enough and that she wouldn't relinquish her freedom by shackling herself to fools like them. Varimathras replied by attacking Sylvanas' base but the banshee used her surgical tactics to fight back and used her banshee power and her banshees to boost her once-low-numbers and succeed in putting the dreadlord against the wall. Varimathras surrendered and offered his services to Sylvanas in exchange for his life. She agreed, but kept him on a very tight leash.

At this time the Plaguelands civil war was already escalated. Opposing Sylvanas' Forces were the Alliance resistance, the Scourge and the Dreadlord Insurgents.

Later the Dark Lady used Varimathras to obtain much information about his nathrezim brothers in order to plot an attack on Detheroc's forces base. Detheroc, instead of eliminating the Alliance resistance and their leader Othmar Garithos had mind-controlled the Grand Marshal and his forces, boosting his own with them.

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 to recover their lost lands. Unbeknownst to him, however, Sylvanas had no intention of giving them their lands back.

One day before the final confrontation, Sylvanas gathered her followers and discussed the next steps and the alliance with Garithos' humans. Lydon was among them.

Sylvanas, Varimathras, and Garithos led a final assault against Balnazzar's center 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 supposedly complied with the task, but unknowingly to Sylvanas, the dreadlord left Balnazzar alive to hide later. And so, Garithos then demanded that Sylvanas leave immediately, grossly and coarsely stating that he wanted "her wretched animals out of his city". However before he could complete his threats, Sylvanas ordered Varimathras to kill him as well, to which he quickly and gladly carried out the order.

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, which started building their new bastion and capital to their newly-founded kingdom on Lordaeron's ruined Capital City's sewers: Undercity.

Allies
Sylvanas and her banshees mind-controlled the following:
 * Mug'thol
 * Blackthorn
 * Snarlmane
 * Murloc Puddle Lord
 * Zul'rogg (possibly)

Trivia

 * In the first mission depicting Sylvanas' Forces, they only had access to acolytes, ghouls, banshees, meat wagons, and obsidian statues. While the first units could represent the undead that would later make up the ranks of the Forsaken, the presence of the obsidian statues is more mysterious. In the non-canon RPG, Sylvanas was described as having been able to "tame" 13 black iron golems, which could have been related to these obsidian statues. There is also the possibility that they were supposed to be an early hint at an Old Gods presence in Lordaeron.
 * Sylvanas' Forces only gained "Scourge" undead units like abominations and frost wyrm after Varimathras joined his forces with them. While abominations became a mainstay of the Forsaken armies, it is unknown what happened to the other units.