“You still do not understand -- but you will be made to. All who are called will serve the Dark Lord.”
Isildur is an antagonist of Middle-earth: Shadow of War. He was the oldest son of Elendil and brother to Anárion and is the direct ancestor of Aragorn, who would return 3,000 years after Isildur died to reclaim the throne of Gondor during the events of The Lord of the Rings. As the High King of Gondor and Arnor during his lifetime, Isildur and his brother Anárion jointly ruled Gondor in the South, while their father dwelled in the North.
During the War of the Last Alliance, Isildur cut the One Ring from Sauron's hand after Sauron killed Isildur's father, Elendil in battle, but he refused to destroy it as the ring seduced and corrupted him. Isildur was later killed by Orcs and the Ring was lost in the Disaster of the Gladden Fields. After his death, the Orcs brought Isildur's body to the diminished and vengeful Sauron, who put a Ring on his finger, reviving him as one of his servants and eventually turning him into a ringwraith.
As one of the nine Nazgûl, Isildur was forced to serve the Dark Lord for all time until he was defeated and freed by Talion. The ranger later claimed his ring to survive after Celebrimbor abandoned him and eventually took Isildur's place among the Nazgûl after holding back Sauron's forces for decades.