In Warcraft III, living nerubians greet Anub'arak with a short accusation: “Traitor King.”
Without the backstory, it seems simple enough. A king changed sides, joined the Scourge, and now helps Arthas.
In truth, Anub'arak fought Ner'zhul until his death. His service began only afterward.
Key Events
The War of the Spider
As Ner'zhul consolidated his power in Northrend, Azjol-Nerub became a serious obstacle. The nerubians were immune to the plague of undeath and resisted the Lich King's mental domination. They could not be swiftly turned into another obedient army.
Thus began the long War of the Spider.
Anub'arak was the last king of Azjol-Nerub. Modern lore added a bitter detail to this story: he repeatedly asked the southern kingdom of Azj-Kahet for aid. Queen Neferess refused to intervene, believing the war would never reach her realm.
Azjol-Nerub ultimately fell. Anub'arak died as well.
Ner'zhul simply raised the former king into undeath.
From King to Crypt Lord
After his resurrection, Anub'arak no longer commanded an army of his own. He became a crypt lord of the Scourge and helped destroy the people he had once ruled.
Nerubian ziggurats and other structures passed to the victors. The Scourge adopted so much of their architecture that much of its familiar presence in Northrend was built from the conquered legacy of Azjol-Nerub.
The survivors called Anub'arak a traitor because of what he did after his return.
It was a cruel title. A person—or, more precisely, a nerubian—might loathe his service to the Lich King, but that hatred could not restore the former subjects he had killed.
Arthas's Road Through Azjol-Nerub
In Warcraft III: The Frozen Throne, Anub'arak meets Arthas in Northrend and guides him toward Icecrown.
Their route passes through the ruins of the old nerubian empire. Along the way, living nerubians recognize their former ruler and again call him the Traitor King.
At first, Arthas even assumes they are speaking to him.
It is a brief exchange in the campaign, but for Anub'arak it compresses an entire life into two lines. He marches through his own ruined kingdom while helping the future Lich King reach Ner'zhul's throne.
In Wrath, He Had to Be Killed Twice
In World of Warcraft, players encounter and kill Anub'arak in the Azjol-Nerub dungeon.
The Scourge later raises him yet again. The second battle takes place beneath the Trial of the Crusader, where Anub'arak becomes the raid's unexpected final opponent.
For a character denied peace after his first death, it was a grimly fitting end.
Why Players Remember It
Anub'arak looks as though he belongs perfectly within the Scourge, which makes his origin easy to overlook. A massive crypt lord standing among nerubian ruins seems like a creature that was always meant to be there as a boss.
But those ruins belonged to his own kingdom.
That fact changes the feeling of Azjol-Nerub more powerfully than a long chronicle could. The player descends into the remains of a kingdom and meets its final ruler at the very end—now serving those who destroyed it.
After that, “Traitor King” sounds far heavier than an ordinary boss title.