Demons are a class of creatures from the Age of Faerie who sought to ravage the land, kill all manner of life, and overrun the world without regard to the consequences. This put them in opposition with the Elves and other creatures of Faerie who sought to preserve and cherish the land and its life.
Eventually the conflict between the Faerie creatures intent on preservation and those bent on destruction culminated in a massive war. Those belonging to the former group were both unable and unwilling to annihilate their Faerie brethren completely, and so they ended the war by sealing the Demons within the Forbidding, a realm of darkness where only the cursed dwell. The Forbidding is maintained by the Ellcrys, a sacred Elven tree.
War of the Forbidding
The Demons threatened the Four Lands when the Ellcrys began to die in the time of the Druid Allanon, and despite the united efforts of the Elves, Trolls, Dwarves, and the Bordermen of Callahorn, those battling the Demons nearly lost the war. The Demons had almost made it to the Gardens of Life where the dead Ellcrys resided when Amberle Elessedil, aided by Wil Ohmsford and the Rover girl Eretria, was transformed within the Gardens into a new Ellcrys, banishing the Demons into the Forbidding once more.
High Druid of Shannara
With the help of mutinous Druids who had turned against their controversial leader Grianne Ohmsford, the Demons attempted to escape the Forbidding once more.
Dark Legacy of Shannara
Centuries after Amberle Elessedil was transformed into a new Ellcrys, the tree started dying again. Under the leadership of the Straken Lord Tael Riverine, the Demons attempted to escape the Forbidding once more. They were ultimately banished back to the Forbidding after Arlingfant Elessedil became the new Ellcrys. Just before they were banished, Grianne Ohmsford in her reincarnation as the Ilse Witch had killed the Straken Lord and taken his place as leader of the Demons. Because the Ilse Witch had chosen to become one of the Demons, she was banished along with them when Arlingfant was transformed.
Originally described as a dark limbo world, the Forbidding where the Demons live is expanded in later stories and is described as a dark mirror world of the Four Lands, a desolate place where little grows and there is no sun, moon, or stars.
The Demons call themselves Jarka Ruus, which means "banished peoples" but which they translate as "the free peoples."
Besides a few unique and especially powerful Demons such as the Dagda Mor, the Reaper, the Changeling, and Tael Riverine, there are also Demons that exist in huge numbers. These various species include:
Demons of the Word/Void and Genesis of Shannara Series
The prequel novels of the Word/Void and Genesis of Shannara series contain demons (not "Demons") of an apparently different origin that seek to destroy the world. They are humans that have sold their souls in service to The Void.
These demons are described in Armageddon's Children as being a new demonkind, born from humans and distinctly separate from the those of the Age of Faerie. When they first accept the offer to join the Void, they begin to shed their human-form, becoming less and less human. Eventually, they are stripped down to a hard, winged husk similar to a bat, which becomes their "true-form." These demons covertly work to cause discord among humans, striving to set into motion the events that will cause the Great Wars that will eliminate most of humanity.
Eighty years later, demons dominate the planet, enslaving and experimenting upon the remaining humans. These demons command legions of once-men, humans who join the demon army and grow insane. The demons even begin to breed new demons by experimenting on children. Most demons retain their human appearance and have the power to subliminally influence others in addition to a greatly extended lifespan. A special form of demon is a Changeling, which are identical to regular demons but for their ability to effortlessly and rapidly change forms.
However, these demons have a number of weaknesses. Their constant need to kill humans and yet coexist with them eventually tears them apart, and eventually they self-destruct. They also have a consuming desire to possess the magic of other creatures, and they often confuse this need with love, furthering their madness.
Some demons prefer to cast away their human appearance, assuming a more lethal demonic form but forgoing the ability to blend in amongst the populace. In this form, they take on a dark, liquid form and blend in with the shadows.
The most notable of the demons of this era is Findo Gask, who is unlike his contemporaries in that he kills and triggers chaos out of pure curiosity, not hatred. He is the primary antagonist of Angel Fire East and the Genesis of Shannara trilogy. Other notable demons in these series include Delloreen, Penny Dreadful, Twitch, the Ur'droch, Stefanie Winslow, and the otherwise nameless titular demon of Running with the Demon.