"Finally, I've seen the light. I can see the fear in the face of the refugees. I felt pity for them and swore to fight against the scourge, but now I've seen the light. Now I remember an older and much more important oath. I've seen the light and now I'm ready to embrace the darkness."
-Last entry in the diaries of the paladin Sir Alpert of Lordaeron
It's said that two months prior to the fall of Kul Tiras, a drifting brigantine appeared near the coast of Kul Tiras. When the crew of the rescue ship boarded the derelict, they found a line of impaled sailors on the deck. It's unknown what was under the deck, as the rescuers' captain ordered to burn the ship as soon as he returned from inside. When the crew returned to the capital, Boralus, they refused to talk about the matter, even in presence of Lord Admiral Tandred. At the same time, an unnamed warrior who identified himself as "The White Knight" arrived the city and sworn to Lord Proudmoore that he would put his sword at the service of the citizens of Kul Tiras if the enemies of Lordaeron were to attack the island nation. Not knowing the secret meaning behind these words, Tandred accepted his oath.
Two weeks after the incident of the cursed derelict, it was discovered that some young men of the capital were breaking the curfew law, wearing black tunics and carrying symbols of dark meaning. Refusing to answer any questions, they were publicly punished and then jailed. Some of them commited suicide, others went into a trance and most of them simply started going mad and shouting in fear from monsters that only existed in their minds.
Fearing that the cult of the dammned could be behind the young men's actios, Lord Admiral Tandred ordered to transfer troops from other cities of Kul Tiras to Boralus to increase the capital's security, but that decision was the one that sealed the fall of the island, as the true members of the cult of the dammned were operating in the minor cities of Kul Tiras, where people started disappearing since then.
When word of the constant disappearance of citizens came to Lord Proudmoore, he ordered the immediate returning of the troops he mobilized to the capital, but it was too late. In only one night, most of the citizens of Kul Tiras except for the habitants of the capital were turned into undead and so were the soldiers that went to investigate the fate of the cities. A week after that, there weren't any living beign outside of Boralus' walls.
The army of undead soldiers and peasants advanced to the capital and laid siege to it, and in the top of the walls was the white knight. When all the undead of the island were gathered around the city, the white knight assasinated mercilessly the guards of the main gate and destroyed its hinges. Then, he dispatched his armor and crossed the field between the gates and the undead completely naked, while the spell that covered his true identity started to disappear, revealing the pale of death in his skin, the white of his mane, the scars that covered his body and the open wound that went through his abdomen to his back. When he arrived to the army, a few acolytes put him on an armor more fit for a scourge general while a ghostly sword appeared in his hand.
By the time the white knight was ready, the soldiers managed to put the gates back in their place, but Tandred knew that they wouldn't resist even the first wave of undead so he ordered the complete evacuation of Boralus' citizens while the soldiers held back the relentless horde of undead.
When Proudmoore was about to board his ship, he found the white knight in his way. Cursing the knight for betraying his oath, Tandred pull out his sword, but his opponent let him go while he smiled and answered him that the scourge were not the enemies of Lordaeron, but the serveants of its only rightful king and that the greatest service he could offer to the citizens of Kul Tiras would be to free them from the burden of life.
After the complete evacuation of Boralus, Tandred decided that the fall of the city wouldn't mean the fall of Kul Tiras, and so he claimed that the deck of every Kul Tiras ship would be sovereign territory of the nation, being his ship, the Windbreaker, the new capital.
Approximately half of the surviving citizens became refugees of the remmnants, while the rest of them became sailors and enrolled into the Kul Tiras navy.