Imshael is an ancient and powerful desire demon. It belongs to group called "The Forbidden Ones". It's the true main antagonist of novel "Dragon Age: The Masked Empire" and a minor antagonist in "Dragon Age: Inquisition".
“The first of the magus cast themselves deep in the Fade in search of answers and power, always power. They found the forbidden ones –Xebenkeck, Imshael, Gaxkang the Unbound, and The Formless One. Many conversations were had and much of the fabric of the world revealed. And thus the magic of blood was born.” ―Unknown mage
Dragon Age: The Masked Empire
Elven clan Virnehn summoned Imshael and imprisoned it in a magical circle in order to learn about Eluvians but the demon refused to help. Imshael manipulated Ser Michel de Chevin by blood magic and created Keystone for him to use Eluvians. Michel believed destroying the circle will send the demon back to fade but it only released it what caused bloody anihilation of the elven clan that summoned Imshael. Ser Michel de Chevin swore to terminate the demon to atone for death of the clan
Dragon Age: Inquisition
Imshael allied with Corypheus and helped in growing red lyrium for his red templars. The Inquisitor can kill it.