His real name was Leareth.
Mellar was born a crossbreed. His mother was herself a Dragon-Dzur crossbreed, and his father was a member of House Jhereg. Because of the stigma the empire places on crossbreeds, Mellar was a virtual outcast, looked down upon by everyone. To get his revenge, he devised a plan to humiliate all three houses that had contributed to his heritage, and conquer each of them at their own strengths, to add insult to injury.
In his youth, Mellar worked freelance for the Jhereg as an assassin, usually taking relatively small or mediocre jobs. He saved his money while he worked on his plan.
A preliminary stage of his plan was to establish a relationship with an adept witch, and use him to ensure the wife of Lord Onarr was infected with a deadly disease, among the various plagues present in the Empire during the Interregnum, and then pay the same witch to cure her just when she was about to die. Onarr was grateful enough that he agreed to train Mellar in the art of swordfighting. Mellar became sufficiently proficient that he was able to defeat seventeen Dzurlords in combat (including the Dzurlord, Lord Keleth), which enabled him to become a member of the House. After two years, he gave up all his titles and joined the House of the Jhereg. This caused a scandal within House Dzur. The House of the Dzur, formerly insulted that a mongrel could fight the best the House of the Dzur could offer and establish himself as a member of their House - were outraged enough to let Mellar know, in no uncertain terms, that they prepared to kill him to avenge this insult. Mellar responded by threatening to make sure the information on the Dzur was released in the event of his death.
As a member of the Jhereg, Mellar rose quickly through the ranks, and in ten years managed to take out Terion (although Terion was apparently not permanently killed), and occupy his place on the Jhereg Council. At the time of Jhereg, Mellar had just absconded with 9 million gold in Council operating funds, effectively crippling a large portion of the Jhereg activities. Far more deadly, however, was the risk to the Jhereg reputation. It is not impossible to steal from the Jhereg; the House's power is rooted in the absolute assurance that it is impossible to survive to enjoy it.
After both humiliating House Dzur and House Jhereg, Mellar turned his attention to playing the Jhereg assassins and the legendary pride of a Dragonlord off against each other to manipulate one into triggering a Dragon-Jhereg war. Ten years prior to his fleeing the Jhereg, Mellar had stolen a book on Elder Sorcery from the library of Morrolan e'Drien, a Dragonlord. Morrolan, who had suspected a Jhereg thief of lifting the book, knew enough of the method Jhereg operate not to accuse Mellar of stealing the book, since he would have done it as a favour to the stealer, in exchange for keeping his identity secret. As a token of his generosity, Morrolan, who is rather predictable in these matters, offered Mellar sanctuary for seventeen days within Castle Black, as Morrolan's guest.
Once a resident in Castle Black, Mellar knew Morrolan would not allow him to be harmed, even though he was insulting him, as well as all the House of the Dragon, by using a Dragonlord to protect himself against hired killers. All guests in Castle Black have protection as per an oath taken by Morrolan, and anyone—even an entire House—who would violate that protection would be subject to Morrolan's wrath. Mellar also knew that the Jhereg could not countenance the impression that there was any force in the world that could thwart their vengeance; to be publicly unpunished for even seventeen days would be an unacceptable show of weakness on the part of the House of the Jhereg. Consequently, the Jhereg would either have to find a way to kill him without violating Morrolan's oaths, or simply attack him while he remained under Morrolan's protection, which would inevitably start a crippling war between the two Houses.
The Jhereg attempted to achieve the former tactic, by bribing a castle guard, replacing him with an imposter, and targeting Morrolan - thus denying Mellar his status as a guest and setting him up nicely. Due to the fact that the Demon had allowed Vlad to cooperate, before determining that he would not take part in a plan that would violate Morrolan's oath, Vlad managed to catch up on events and enlist the help of Aliera e'Kieron to revivify Morrolan and deal with the assassin before he moved on Mellar.
Later, Vlad managed to defeat Mellar's plan and avert the war by way of a series of complicated maneuvers carried out while Morrolan was on guard inspection, which involved using Kiera to switch two key daggers on Mellar's person, located on the right waist sheath, and left wrist sheath, with matching Morganti blades, and using himself and Loiosh to provide a distraction in the banquet hall where Mellar resides in order to position himself so that he can replace the bodyguard to Mellar's left, then using Aliera, also a guest within Morrolan's fief, to temporarily put down both bodyguards, then engage Mellar in combat, and forcing him into a situation where he could employ the use of either one of the daggers without killing her permanently (thus violating any claim to protection under Morrolans' rules), assuming he still has his daggers, by way of a deliberate opening on Alieras' part (Aliera was good enough a swordsman to stimulate the moves correctly, and Mellar was too good a swordsman to miss the obvious opening), then using Cawti to kill the bodyguard to Mellar's left, once Mellar kills Aliera, and, at the same moment, Daymar to cast an illusion on himself in order to impersonate the dead bodyguard. After killing Aliera with the Morganti dagger, Mellar panicked and turned to Vlad to teleport him out, who did not respond. Mellar then turned to the other bodyguard, who complied, teleporting them to the jungles west of Adrilankha. Before the teleporation, Mellar had already suspected Vlad of being an imposter. Vlad is able to put the other bodyguard out of action, before Mellar begins his attack. In the ensuing fight, Vlad desperately uses a variant of the witchcraft spell he used to his familiar as an egg, instead bringing a fully grown jhereg which distracts Mellar in time to prevent him from finishing Vlad off, forcing him to fend her off. Vlad uses the last of his energy to insert Mellar's Morganti dagger into his left eye.