Illenai was probably one of the Daemonfey, and so hails from the High Forest. His history before coming to Beldin is unknown. He first became known as an Aurilite spy who had infiltrated Siluvan. Having wormed his way into the favor of Lord Aravilar Jadys'Varine, he came to Beldin as ambassador of Siluvan soon before the Siege of Beldin. During that visit, he demanded that Beldin exile all the drow of the city before Siluvan would aid them, thereby inflaming the tensions between Beldin and Siluvan over the matter of the drow.
Illenai was eventually revealed as the shapechanger and spy that he was, and driven out of Siluvan. He began working on and off with the Sharrans, his first project being the modification of the mythals to plunge the valley into darkness and create the Shroud to hide the valley from foreigners--mortals and gods alike. He did not succeed in 'darkening' the valley due the efforts of Zilvra Vrinn, who switched one of the components with something innocuous. Illenai was then confronted while he was performing the ritual on the mythal. He completed the spell, but was slain. He escaped by means of a contingency spell.
Later, Illenai was tracked down by Heniandrar and Ronin Eininfar in the Aelfwood, and killed again. This time, they burned his body. Heniandrar recovered one of the Shattering Swords from the corpse, and returned it to the tomb of Aranas Jadys'Varine from which it had been stolen.
Some years later, however, Illenai was raised by means of a wish spell by the Sharrans. In order to obtain this favor from Shar, the Sharrans had to recover the artifact the Breath of Shar, which they tricked the adventurers into finding for them. Once it was delivered, Illenai was raised from the dead, and became beholden to them to a certain extent. Since the Shroud had been 'disabled' by the Mind Flayers who took over the city during the Luskan Invasion, Illenai first worked to put it back in place.
After paying his debt to the Sharrans (at least in his own mind), Illenai began working more for himself. He researched a method to draw power directly from the mythals. To an extent he succeeded, but his connection was severed by the efforts of the Guild.
For a time, he was also working with the Yuan-Ti, but they became convinced that he was taking advantage of them, and so that relationship ended. There were also numerous incidents of random mischief caused by Illenai.
During the reign of Malark, Illenai participated in the attack on the enclave. He escaped alive due to a contingency spell. He seems to have also worked with the Oracle, since he was instrumental in the kidnapping of Twilla Sweban.
After this during the time in which the mythals had failed and the demon gate opened, Illenai once again apeared this time at the forefront of balor generals weilding soul capturing swords. Illenai was found in the city of Siluvan moments before he was going to begin destroying it and was slain by Ronin Eininfar with the soul capturing blade of his balors. The blade was then handed over to King Aravilar Jadys'Varine .