Solanagen-based entity

The Solanagen-based entities were a species of Solanagen-based lifeforms who could only live in subspace.

In 2369 the lifeforms attempted to create a pocket of their space inside usual space and began to abduct members of the crew of the USS Enterprise (NCC-1701-D) to perform medical experiments on them, trying to learn how to remodulate their cellular energy states in order to survive outside of their tertiary subspace manifold.

Abductees included Data, Kaminer, Geordi La Forge, Worf, Sariel Rager, William T. Riker and Edward Hagler, the latter of which died when the entities replaced his blood with a liquid polymer.

The Enterprise crew were able to close the spatial rupture the entities used to pull victims into subspace through when Riker took a homing-device with him in one of their abductions. The devise allowed the crew to target the tetryon particles being used to from the rupture and close it. (TNG episode: "Schisms")

The harrowing experience lay within Riker's mind for years to come and was one of the things he remembered during flashbacks caused by the Orb of Memory in 2376. (DS9 novel: Avatar)