Of all the threats from beyond the Veil, few are more insidious and deceptively deadly than the desire demon. Far more intelligent than the bestial hunger and rage demons, and more ambitious than the demons of sloth, these dark spirits are among the most skilled at tempting mages into possession. Their abilities to affect the mind allow them to assume disguises and even alter the environment to their purposes, not to mention the great strength and speed they possess if they should have to resort to more physical means. Most often a desire demon will attempt to bargain its way to freedom if overpowered.
See: Dragon Age: Origins