Archimonde's defeat in the world tree marked the defeat of the burning legion, his death was supposed to bring peace to all kingdoms, but it was not, one of his creations had survived the Lich King.
The Lich King was the jailer of the living dead, without the presence of the hordes of the dead destroying everything in his path, he desperately needed a champion to free him without destroying him in the process.
Prince Arthas was chosen and betrayed everything he knew in the name of the Lich King, but the closer he was to freeing his lord, the more enemies.
The oldest guardians preceded the end of life if the Lich King were to be freed and they began a campaign against Arthas and the undead legion, In battle Prince Arthas died and with the many champions from all the eastern kingdoms. Victory was near, but the Lich King uses all his power to revive the fallen champions, including Prince Arthas, who became a simple puppet, but with even stronger power. The champions fighting for life had no choice but to retreat, to the only fortress they had left standing and the only place where the dead had not disappeared. The tree the world In this bastion all the champions gather to face their enemy, the victory was not certain since the Lich King promised eternal life after death and power, many voluntarily surrender, but fate was not written, when the world tree was about to fall. Champions of all races rose to defend him, thus halting the invasion of the undead.