Plaguelands civil war

The Plaguelands civil war was a conflict between several Burning Legion-controlled factions of undead led by the nathrezim Varimathras, Detheroc, and Balnazzar, a faction of independent undead (that would later be known as the Forsaken) led by Sylvanas Windrunner, and the Alliance resistance led by Lord Othmar Garithos. Due to Illidan Stormrage's ritual with the Eye of Sargeras he was able to injure the Lich King in Northrend, the Lich King, in turn, urged Arthas to return to Northrend to defend him. With Arthas gone, and the Lich King gravely weakened, the aforementioned factions rose up to take control of the power rift left behind. The war ended with the deaths of Balnazzar and Lord Garithos, and left Sylvanas and her lieutenant Varimathras as the sole power over much of former Lordaeron.

A new power in Lordaeron
Though Illidan's ritual was disrupted in time by Maiev Shadowsong, Malfurion Stormrage, and Kael'thas Sunstrider, he was able to injure Ner'zhul in the Frozen Throne, causing his power to seep out. Ner'zhul communicated with Arthas several times, and urged him to return to Northrend to defend him. While Arthas was preparing to leave, he was beset by the dreadlords and their forces. Separated from Kel'Thuzad and gravely weakened, he was grateful to find many undead loyal to him who helped his flight from the Capital City. Arthas wasted no time in departing and appointed Kel'Thuzad to rule the Plaguelands in his stead. Due to the Lich King growing weaker, the Scourge overall was in a much weaker state, and Kel'Thuzad and the Scourge remained largely passive and absent for the duration of the civil war.

In the wake of Arthas' departure at Hallow's End's day, Sylvanas, and some other strong-willed undead were able to free themselves from the Lich King's grasp, while the Dreadlord brothers Varimathras, Detheroc, and Balnazzar took control of the majority of undead forces. They strongly believed that the Scourge belonged to the Burning Legion, and attempted to maintain a Legion presence on Azeroth despite their great loss on Kalimdor. As rivalries between the undead factions grew stronger, Lord Garithos dreamed of reforming the kingdom of Lordaeron, and joined the fray with what remnants of Lordaeron he could gather. Despite the defection of the blood elves, Garithos' Alliance resistance forces were able to carve a foothold for themselves by retaking Dalaran. As the dominant force at the start of the war, Varimathras, at the behest of his brothers, extended a formal invitation to Sylvanas and her forces to join them, promising her that they are the future of the Plaguelands. Sylvanas declined, and steeled herself to fight a much larger foe.

The civil war
On the onset, Sylvanas was at a great disadvantage, with her army consisting largely of ghouls and banshees and not much else. She had her banshees possess the local bandit group leader, Blackthorn, and the ogre leader Mug'thol, to bolster her armies and devastated Varimathras' forces. Before Sylvanas could kill him, Varimathras pleaded with her to spare his life in exchange for information on his brothers' forces. Sylvanas complied, making Varimathras her lieutenant but keeping a close eye on him.

Working off of the information Varimathras provided, Sylvanas learned that Detheroc had entrenched himself in the western Plaguelands and has used his powers to mind control Lord Garithos and subdue his forces. Sylvanas faced a dual threat in Garithos and Detheroc's forces and used subtlety and deception to sneak her way past guards and establish a presence behind their city gates. When the forces of Detheroc and Garithos awakened, they clashed with Sylvanas, who was able to devastate both forces. After Detheroc was destroyed by Sylvanas and Varimathras, the mind control spell put on Garithos and his forces was lifted. Sylvanas approached him and offered to join forces to defeat their common enemy - Balnazzar and give him his lands back. Garithos reluctantly agreed and rallied his armies. Varimathras questioned Sylvanas' decision, and she informed him she had no intention of returning Lordaeron to the humans.

Sylvanas, having defeated Detheroc, and joined forces with Varimathras and Garithos, marched on the Capital City, where Balnazzar resided. Despite being the last dreadlord, Balnazzar had the largest army and had opened gateways to summon demonic allies. Sylvanas organized a two-pronged attack on the capital city: Her and Varimathras would attack from the front, while Garithos would attack from the rear. Upon encountering Balnazzar, Sylvanas taunts him about enslaving his brother and tearing the other to shreds. Balnazzar expresses outrage at Varimathras's betrayal when encountering him, and when encountering Garithos, he reveals that he was the mastermind behind the mind control, not Detheroc. Eventually, Sylvanas and Garithos are successful, and they confront a defeated Balnazzar. Sylvanas orders Varimathras to kill him, to which he reluctantly obliges. She then orders him to kill Garithos as well. She formally declares her defection from the Scourge and names her forces the "Forsaken," vowing to slaughter all who stand in her way. Sylvanas's declaration and the deaths of Balnazzar and Garithos mark the end of the civil war.

Aftermath
Sylvanas is able to successfully consolidate Forsaken rule over Tirisfal Glades and Silverpine Forest, but was ultimately pushed out of the Plaguelands. The Forsaken are largely split into two major parties: those loyal to Sylvanas and those loyal to Varimathras. She joins the Horde shortly after in an attempt to create more allies for herself, in exchange for the Horde having a presence on the Eastern Kingdoms. A relationship that began as being exclusively mutually-beneficial quickly evolved into a much stronger bond, as the Forsaken became a crucial part of the Horde, playing major roles in most of their campaigns until Sylvanas' ultimate ascension to Warchief.

Varimathras initially maintained his loyalty to Sylvanas, but during the Battle of Angrathar the Wrathgate, he and Grand Apothecary Putress, along with his loyalists, stage an uprising in which Sylvanas is kicked out of the Undercity and Varimathras renews his loyalty to the Burning Legion. He and Putress are both stopped and Varimathras languishes in Antorus, being punished for his continuous failures.

The status of the remnants of Garithos's forces is unknown. Seeing the free-willed Forsaken occupy their former capital, the remnants of the Knights of the Silver Hand who didn't join with the High Marshal renewed their assaults on the undead. Balnazzar eventually infiltrated their ranks as racist dissensions splintered the resistance's leadership, leading to a split between the former knights into the two orders of the Scarlet Crusade and Argent Dawn. Balnazzar and Varimathras kept in contact, working together to protect their interests.

With Arthas's ascension to the Frozen Throne, the Scourge regained their power, and Kel'Thuzad wasted no time in reclaiming the land he lost during the civil war. He ruled over the Plaguelands from atop his floating Necropolis, Naxxramas.

The power gap left by Arthas and the weakened Scourge was ultimately filled by Sylvanas for a short while after she won the civil war, but with the Scourge renewed in their strength, they returned as a serious threat in the region, with only the Forsaken, the Argent Dawn, and the Scarlet Crusade to challenge them.