| To understand the demonic one must first understand its opposite number.|
There are many names for it: the One, the All-God, the Font of Creation. No single name is correct, but for purposes of picking one we will go with the Divine. It is the power literally above and beyond Holiness, the raw stuff of creation and motion that gives rise to everything else. The Divine acts by writing reality. It cannot toss a ball, but it can change its coordinates in four-dimensional spacetime to situate it somewhere else.
The Divine is many things. Noteworthy, the end-goal of the Necromancer's quest for transcendence, the goal of stealing the flame of Divinity and delivering it to a humanity that can scarcely begin to understand the blood-drenched gift.
This is an extended digression for a reason. The Divine is everything an Elanthian knows as reality, and when venturing outside the reach of the Divine (such as the Plane of Probability and other, distant planes) things become weird and dangerous very quickly. There is nothing safe, nothing productive, no home without the Divine.
Demons think so, too.
A demon is a creature that, by its metaphysical physiology, an anti-Divinity. Demons are drawn to the Divine in the same way that a starving man is drawn to a good meal. They are not necessarily morally evil, on the whole of it, but they do embody at their root the desire to scoop out the living heart of the Plane of Abiding and eat it.
The shape and scope of demonic creatures varies wildly. Adventurers are used to minor evil spirits demonic manifestations that are "human-sized" and generally killable by normal means. However, we rarely think of them when we say demon. What about the Maelshyves and Hungers of the universe?
In short, it's easiest to think of them as gods.
When the Hunger manifests, it is not "the Hunger" as a whole and complete entity, but a small avatar or metaphysical appendage of an entity that cannot be fully described in three-dimensional space. When you "kill" a Maelshyve manifestation, you have essentially chipped her fingernail. The greatest demons swirl outside the Plane of Abiding, probing it and inserting tendrils into it whenever they can. If a tendril is sliced off, that doesn't seem to impact the amorphous demon-god very much; it simply picks another time and place to try again.
The nature of demons also vary, though they vary within a theme. Broadly, every demon manifests through some perversion, what we can poetically think of as the demon's horror. Maelshyve' for example, manifests as an anti-god, having her minions construct elaborate temples and worship in her name as a god-that-cannot-be. The Hunger manifests as a natural law gone wild, entropy invested with supernatural speed and malice. The Hunger acts more like a natural force than a malefic intelligence.
There are an unknown, perhaps endless number of these demon-gods. They probe for spiritual, moral, and metaphysical weaknesses to exploit and it takes only one of them succeeding to tear down the walls of creation and kill everyone, everywhere, for all time.
Sometimes people ask how the Temple manages to keep so zealous years after the Lyras incident. To someone who understand the Plane of Abiding is under siege, it's more a question of whether they're zealous enough.
"Perinthia's astronomers are faced with a difficult choice. Either they must admit that all their calculations were wrong ... or else they must reveal that the order of the gods is reflected exactly in the city of monsters." - Calvino Italo