Some demonic spell ideas:
World of the Blind: The forces of darkness cover the victims eyes with shadow, making them unable to see what's going on around them and forcing them to rely on their other senses. This crippling effect lasts 1 minute for the first circle and thirty extra seconds for each additional circle. (Strangely, when cast upon those who are already deaf and dumb, this spell has been known to make it's victims skill in pinball drastically increase to inhuman proportions.)
Fear and Self Loathing: The victims mind becomes plauged by fear and self-doubt for approximately ten minutes. The amount of circles used determines the strength of the effect. 1 circle being a minor distraction that makes it slightly difficult to focus on attacks and defenses, while a full 6 circles can be completely paralyzing to even the most couragous and strong willed warriors.
Evil Dead: In which one turns a corpse into a temporary bridge which lesser demons can utilize to enter this world and inhabit the corpse. These entities are not as powerful as summoned demons. But they are hungry for souls and eager for battle. When they inhabit the body it becomes an undead berzerker. The demon doesn't care wether the undead host dies again, sometimes dismembered limbs keep fighting until the effect fades. 1 Circle will last for 5 Minutes. Each succeeding Circle adds an extra 3 Minutes.
Invocation of Heavy Metal: This spell causes an unholy racket of demonic music to sound throughout the surrounding area for anywhere from 2:59 to 13:02 minutes, depending on the length of the song. This enables the demonic knight (and any of their allies who happen to be ready to rock) to enter a mystical state of battle meditation known as "The Zone". While in The Zone all demonic knights/allies who rock receive a significant boost to their fighting ability depending on how loud the music is. More circles = louder. However, since Heavy Metal can only be played loud, this spell can only be cast with 3 or more circles.