The Nathrezim, better known as Dreadlords, are an ancient and malevolent race of demonic beings in the Warcraft universe.
Not much is known about their history, only that they are one of the few races of the Burning Legion to have been demonic from the start (most of the Legion races were corrupted or enslaved). They were also responsible for the corruption of the Titan, Sargeras, and vowing their loyalty to him. The Nathrezim assisted Sargeras in continuing to corrupt other races, bringing them into the Burning Legion.
The Nathrezim are masters of disguise. Their ability to posess their victims has helped them destroy their enemies from within. They also seem to be vamparic, though instead of feeding on blood they seem to feed on souls.
To kill a dreadlord is incredibly hard. Should they be "killed", their bodies merely fade away, allowing their souls to regain their physical form in time. In order to periminently destroy a dreadlord requires one to use an item or other magical source far greater in power than the targeted Nathrezim. This has only ever been confirmed to have happened once when Illidan Stormrage used the Skull of Gul'Dan to kill the dreadlord leader, Tichondrius.
Tichondrius - The former leader of the Nathrezim. He was responsible for the corruption of Sargeras, thus responsible for the creation of the Burning Legion itself. He was killed by Illidan Stormrage and the throne was given to his brother, Anetheron.
Anetheron - Brother of Tichondrius and the presumed leader of the Nathrezim. When the Archlich, Kel'Thuzad, summoned Archimonde to Azeroth, the demon lord gave leadership of the Scourge to Anetheron and the lich, Rage Winterchill. Anetheron was presumably killed at the Battle for Mount Hyjal.
Mephistroth - The presumed leader of the Nathrezim. He was only ever once seen in the game series, and that was during the meeting between Tichondrius, Mephisroth, and Anetheron. After Anetheron's possible death at Hyjal, it is believed that Mephistroth may be in charge.
Balnazzar - Nathrezim leader in the Eastern Kingdoms. He was subdued by the Forsaken, returnning and disguising himself as the leader of the Scarlet Crusade.
Mal'Ganis - One of the two Necromancers who created the Scourge, alongside Kel'Thuzad. Mal'Ganis was presumably killed by Arthas, but returned in disguise as leader of the Scarlet Onslaught.
Varimathras - Former commander of the Scourge forces in the Eastern Kingdoms. Betrayed the Legion to work as Sylvanas Windrunner's majordomo, but eventually revealed himself as still loyal to his former allies and worked to summon the Legion into the Undercity.