Neslar was the Third dark lord of Middle-earth who in the First Age was a elf who was taken by Morgoth and made into a dark creature who climbed in rank almost as high as Sauron before being driven to the dark lands by Earendil. He is also the Brother of Galadriel.
In the fourth age he invaded Middle earth killing Aragorn and being captured and thrown into the void by Manwe of the Valar (it was at this time that Elves started to revisit middle earth on occasions). In the rule of Eldarion Neslar made his second invasion (after somehow escaping from the void triggering the great magic disorder)where he reestablished his kingdom of Neslina before finally being killed by Eldarion's son Elderenon. Starting the 5th age destined to be the shortest age of all. His ghost returned to the void, where he showed Sauron and Morgoth how to break the door, and they all escaped starting the war of the 5th Age.