Lord Xavius was a powerful user of magic, and the high councilor to Queen Azshara in Warcraft. He was later turned into the first satyr by Sargeras, before ultimately being transformed into the dreaded Nightmare Lord.
Xavius was renowned for two things in particular: his skill at subtle political maneuverings and his lack of eyes — he replaced his eyes with magic black crystals that bore streaks of crimson. He is perhaps the night elf most responsible for the Burning Legion's interest in Azeroth. It was Xavius who first became convinced that Sargeras, ruler of the Burning Legion, was a god sent to deliver Azeroth. Xavius also convinced the night elven queen Azshara of the necessity of turning the will of her mages, the so-called "Highborne", towards creating a portal into the Twisting Nether through which Sargeras and the Legion could enter into the world. Xavius was successful enough in his endeavors to welcome several ranking members of the Legion into Azeroth, including several felguard, Hakkar the Houndmaster, and Mannoroth, before the intervention of the young druid Malfurion Stormrage. As suggested by Azshara, Xavius executed a plot to block any non-Highborne magic user from accessing the Well of Eternity. This allowed the highborne to channel the extra energy gained into making the portal to the Twisting Nether strong enough for Sargeras to cross through. Xavius's plot was defeated by Malfurion, who after entering the Emerald Dream, destroyed the spell array holding the block and the portal; in the battle that ensued Malfurion destroyed Xavius completely.
Xavius’s master, Sargeras, was unhappy with Xavius, and he faced punishment for his failures. Sargeras tortured Xavius’s disembodied spirit for a time, but eventually he showed a form of mercy. He decided that Xavius could be of further use after all. He returned Xavius to life, but not as he once was. To mark the traits Sargeras valued in him, and to brand him forever as an ally and servant of the Legion, Sargeras transformed and disfigured Xavius's body. He created a new body for Xavius: a body with cloven hooves, shaggy goat legs, goat-like horns, razor-sharp claws, and a long, leonine tail; a satyr.
As the first of these beings ever to inhabit Azeroth, Xavius is considered the progenitor of the entire satyr race. As such, variations of his name are found frequently in satyr settlements (i.e. Xavian in Ashenvale). As a satyr, Xavius retained some resemblance to his former self along with the typical characteristics of all satyrs. Xavius was unique, however, in that he retained the black and ruby eyes he had possessed as a night elf.
Xavius had a two-pronged mission when he returned. First, he was to begin converting as many Highborne as possible into satyrs (The first was a personal friend of Xavius, a dedicated Highborne named Peroth'arn), a task he accomplished with zeal by promising the unfortunate elves power and a place at the right hand of Sargeras. The second objective Xavius was to complete upon his return was to bring back to Sargeras the one who had destroyed the portal, Malfurion Stormrage. This second objective proved to be far more difficult and, in a confrontation during one of the most intense battles of the War of the Ancients, Malfurion, with help from a young night elf named Shandris Feathermoon, was able to completely defeat Xavius. Using an arrow shot by Shandris, Malfurion was able to kill Xavius by growing a tree which absorbed his energy and body, which was subsequently sent to the bottom of the sea in the Great Sundering. (The exact description of Xavius' fate in the novel The Demon Soul says, "Xavius was not so much becoming a tree — rather, his body was providing Malfurion's creation with the nutrients and building blocks to make itself.") This was the second time Xavius died at the hands of Malfurion. The victory came at a great cost, however, as during this confrontation Malfurion's love Tyrande Whisperwind (High Priestess of Elune) was captured by Xavius' remaining satyrs. In a furious rage, Malfurion almost drowned the entire night elven army in a storm caused by his own sadness over the loss of Tyrande. Luckily Krasus intervened and stopped Malfurion.
The tree containing the remains of Xavius's soul survived the Sundering through unrevealed means. Though given his later state and presumed allegiance to the Old Gods, it is likely they intervened, much the same way they did for Queen Azshara. Ten thousand years later, Xavius came to lead the Emerald Nightmare as the Nightmare Lord. In this form, Xavius orchestrated the corruption of the Emerald Dream by manipulating Fandral Staghelm into planting a branch of Xavius's own tree into Teldrassil, ensuring that the new world tree would be corrupted from the start. His plans culminated in a daring attempt to bind Azeroth and the Emerald Dream into one plane, seizing control of Azeroth's residents while they slept and giving shape to their nightmares. He nearly succeeded, but ultimately was defeated by the re-awakened Malfurion Stormrage who was assisted by Tyrande Whisperwind, Broll Bearmantle, Thura, Ysera, Eranikus and a host of others. Malfurion sensed however, that while Xavius was strong, he was merely the servant of something even more powerful.
When in the form of the Nightmare Lord, Xavius is described as being a massive, gnarled tree, greatly obscured by the dark fog of the Nightmare. He delights in torturing Malfurion in various ways, both psychologically (such as showing him Tyrande, as the lover of Illidan, being turned into a demon) and physically (slowly turning him into a tree, for example).