Aviss was a once-human woman magically transformed into a bird-like being, who sought the destruction of Waterdeep.
At some point, they went into the Mere of Dead Men, far to the north of the city. There they met with trolls, survivors of the earlier Trollwars waged against Waterdeep. The trolls believed the traitors could bring them victory in a third, and final, Trollwar, and promised them power over the city. Aviss and Fellandar chose to give up their humanity and swore a bond between them. As the trolls watched on, Aviss and Fellandar entered a certain pool where the fell magic of the Mere transformed them into inhuman creatures of raw and primal power.
Empowered, the pair led a new troll army out of the Mere and south toward Waterdeep in the Year of the Thundering Horde, 963 DR. They knew well the city's defenses, and Castle Waterdeep had only just begun construction a month before. However, the great mage Ahghairon and a band of wizards met them and hurled Aviss and Fellandar into a sunless extra-dimensional prison, thwarting the troll's attack. [note 1]
For long ages, Aviss and Fellandar struggled against the door of their prison, seeking a mystic nexus point by which they might break out. Such a point was eventually to be found within the Selûne's Smile tavern in Waterdeep.
One evening on the hottest day of summer in the Year of the Prince, 1358 DR, Vajra Valmeyjar and Timoth Eyesbright arm-wrestled in the Smile. Little did they know that, in the skies above, the constellations of the Woman Warrior and the Centaur were in alignment, giving power to their contest. This power merged with magic sweltering in the heat, widening the point into a rift and thence a door to the extra-dimensional prison, releasing Aviss and Fellandar at last.
Resuming their wish to destroy Waterdeep, they wreaked a path of destruction through the city, killing and maiming folk, destroying buildings, and overturning carts. They were bewildered by the changes to the city, and shocked to realize how long it had been. They were pursued and fought all the way by adventurers Kyriani Agrivar and Onyx the Invincible. Nevertheless, the spell that imprisoned them drew them back toward the Smile, where Vajra and Timoth still arm-wrestled to keep the rift open at Khelben Arunsun's urging. Finally, Luna dazzled the pair with moonlight and Khelben used a telekinesis spell to hurl the villains back in, last seen begging against it.
Aviss was cruel and merciless, though she loved Fellandar dearly.
Aviss was a bird-like woman, with dusky skin, red hair shaped like horns, yellow slitted eyes, long raking talons on her fingers, and a pair of feathered brown wings that allowed her to fly. She wore a gold-hued breastplate, red boots, and a red sash. She spoke in an archaic form.