The "Subspatial Beings" (for the lack of a proper name) are sapient spacefaring humanoids which inhabit the realm of subspace and have a molecular structure based on solanogen; a sub-spatial chemical which appears to take a role analogous to carbon in the Human body.
Virtually nothing is known about these lifeforms, which have a distinctly reptilian look and (at least those which have been observed) dress themselves in metallic hooded robes. Their hands have three long clawed digits and their faces are fish-like, with bulging eyes and a beaked mouth. They speak in a language which apparently cannot be translated by a 24th-century Federation UT, and which sounds like repeated series of rapid clicks.
In the year 2369, a group of subspace lifeforms used advanced technology to create pockets of their own realm within normal space and abduct crew members of the USS Enterprise-D when they were asleep, performing a myriad of enigmatic experiments on them and returning them with erased memories afterwards. One of the abducted, Lieutenant Hagler, died as a result of having his blood turned into a form of liquid polymer.
It was later found that the beings had been attracted by the Enterprise due to modifications made to the sensors by Commander LaForge, which caused the ship to broadcast intense subspace signals. The ship used a graviton pulse to shut the connection which the aliens had built between realms, but not before an enigmatic energy probe was launched into normal space by the subspace dwellers, with unknown destination and purposes.