Riftwraiths are corrupted former Harmonic Weavers. Their history makes them especially troubling: they are not simply unfamiliar intruders, but beings connected to work that once served the integrity of the Weave.
Rifts can bring Fiends into a reality, and Riftwraiths are associated with the Nullweaver scouts. The precise relationships among these threats deserve their own connected records rather than being flattened into a single category of monster.