This article contains SPOILERS. Read at your own risk. This article is incomplete. Look for a completed version in the future.
Before the Great Plague
In his earliest days, Asmodan was a great Elven warrior who eventually rose to be the leader of the Numitari Clan, named after his family. He was an intelligent and charismatic leader who led his people against the forces of the Netherworld at every opportunity, and eventually became the most widely-known Elf on Borendil.
The Aesir could not stop the Great Plague, partly because it was an extremely advanced disease and because of its quick onset. However, they were able to save Asmodan by teleporting him onto one of their ships before he caught the disease. Asmodan reports that five minutes after his arrival on the ship, the Aesir told him that they could confirm the deaths of every Elf on Borendil.
Time with the Aesir
Asmodan spent many years with the Aesir - he was not around to see the emergence of Humans nor the beginnings of their societies. However, he kept up-to-date with that information while volunteering for Aesir experiments into the magical nature of the Elves.
Asmodan was a key figure in helping the Aesir develop arcane means to fight the Great War. The Aesir themselves have no affinity for the use of magic - their ability to fend off the substantial might of the Netherworld comes from their extremely advanced technology alone. Notably, Asmodan helped the Aesir construct a field device that would disallow Nyarlathotep to escape a planet where he had been caught. This resulted in Nyarlathotep's sealing in the Void by the Elder God Vorvadoss.
Return to Borendil
Asmodan longed to return to his home. After the Aesir confirmed that the disease that killed the Elves was no longer in existence on the planet, they allowed Asmodan to return and mingle with the emerging Human societies. To help him remain "in the know" of the events on Borendil (the Aesir assumed that the planet, with its nexus of mana, would be a key location in the Great War), the Aesir gave Asmodan the Knowledge Rune, which contained all the knowledge of a great elven sage before he was killed by the plague. Asmodan is still in possession of the rune.
For many years he wandered the planet and learnt much of its new inhabitants. The Aesir gave him a device that would mask his appearance, so that he would appear to be human. He continued to remain seperate from human affairs and instead became the Aesir's contact for information regarding Borendil. Using the raven avatars Huginn and Muninn granted to him by the Rune, along with his own personal travels, Asmodan was able to keep up-to-date on most events on the planet. He would not become part of something significant until the events of the First Invasion.
The First Invasion
After he received information about an army of demons that appeared in the Seperatist States, Asmodan went to investigate. Indeed, the city-state of Berle had requested aid from the Holy Kingdom of Arduran in fighting an army of invading demons. Asmodan assumed that they originated from the Dismal Caverns, a place that was known to the Elves as a location with strong ties to the Netherworld. He underestimated the intelligence and organized nature of the demons, and upon entry to the Phantom Forest, Asmodan was ambushed and knocked unconscious with a gas grenade.
He was taken to the Dismal Caverns Prison Bloc and remained there for three weeks before the arrival of Antredius and his party. During that time, he used Huginn and Muninn to keep him informed about the happenings outside. When Durth (Black Orb of Death), a member of Antredius' party, was captured and placed in the cell next to him, Asmodan helped him escape but declined the help in return (He claimed it was because he wanted more time to think, and that the food was good; but in reality Asmodan would have had far better luck escaping if he were alone). After the commotion caused by the escapees, Asmodan escaped the prison himself and flew out of the Phantom Forest.
Asmodan flew to a remote location and used his Communicator to contact the Aesir. After some discussion, it was decided that the forces of the Netherworld were trying, once again, to invade Borendil. For the first time since the extinction of his people, Asmodan decided to begin helping in human affairs.
Search for the Runes
Asmodan knew that in order to combat the Netherworld, the Runes of Righteousness would have to be gathered. Since he saw that the Bright Savior Rune was already in possession by Lester (the former human drone that Dantalion tried to use to force the rune into, but became possessed by the spirit of Antredius instead), he thought that perhaps the runes would choose some of the individuals traveling with him. He sent them to speak with High Priest Roland Valtierre in Arduran, a personal friend of Asmodan's. The High Priest was one of the two people (Asmodan himself included) who knew the secret location of the Creation Rune.
While the party was off seeking the High Priest, Asmodan was teleported aboard the Fist of Atlantis, flagship of the Aesir Fleet. He met with Commander Penegal and informed him of the situation. It was decided that Lemuria would be moved to orbit around Borendil and the planet would once again become protected by the Aesir. However, they refused to interfere with any developments on the planet's surface, as their presence would quelch the abundance of mana on Borendil. Instead, they left the surface matters to Asmodan.