Conceptual entities

Engineered beings or weapons (in many cases conceptual entities are both) which exist only as concepts, and have no provable substance at all. There is a reasonably subtle idea, and several cultures involved in the Last Great Time War have had difficulty grasping it. As conceptual entities only seem to affect the minds of their victims, it's often said that the entities are 'made out of pure thought', but this is clearly inaccurate as thought itself isn't a substance. Although many people are determined to think of the entities as telepathic presences, or neurological parasites, or in some cases even as "spirits", in fact it's much more accurate to think of them as nothing more than hostile ideas. They exist by bypassing matter altogether, and instead giving themselves structure inside the meanings of things.

For example, a victim affected by a conceptual entity might be reading a book and suddenly discover there are messages in the text which shouldn't be there, communications from an entity which has (like any good idea) taken root in the invisible connection between the book and the reader's mind. The victim isn't hallucinating these messages.

They have indeed been planted in the book, probably in the exact same font and style as the rest of the text. But the entity hasn't changed the physical nature of the text at all. Instead, the network of understanding which surrounds the reader-- the "topology of comprehension"-- has been used to alter the meaning behind the book. In effect, the entity has wormed its way into culture itself.

The Celestis are, at least in part, a race of high intensity conceptual entities.