The evil king mentioned here is the Witch-King of Angmar, the Lord of the Nazgûl and Sauron’s chief servant. When Sauron returned in the Third Age, he secretly came to Dol Guldur in Mirkwood and gathered the nine Nazgûl to him. Seeing that the northern kingdoms of men were vulnerable, he sent the Lord of the Nazgûl, who was a powerful sorcerer, to destroy them. Establishing a kingdom in Angmar, the Lord became the king of Carn Dûm and became known as the Witch-King. There he succeeded in bringing about the downfall of the Dúnedain, but Isildur's line (heir to the thrones of Arnor and Gondor) managed to survive and continued in secret. With the help of the elves the Witch-King was eventually defeated and driven from his realm.