Robed, clicking beings from a deep subspace domain, built from solanogen-based matter, who secretly abducted members of the Enterprise-D crew in their sleep and performed surgical experiments on them.
These aliens appear in the Next Generation episode Schisms. They live in a tertiary subspace realm and cannot exist in normal space, any more than humans can survive long in theirs. Their attention is apparently drawn in 2369 when La Forge modifies the ship's sensors for a mapping survey, creating tetryon emissions they can follow.
Riker, Worf, La Forge and others begin suffering exhaustion, lost time. Troi brings them together on the holodeck, where they jointly reconstruct a dim chamber with a metal table and a scissor-like instrument. Crusher finds that Riker's arm has been severed and reattached, and one crewman returns with his blood turned to a liquid polymer and dies.
Meanwhile a subspace rupture is growing in a cargo bay. Riker lets himself be taken while carrying a homing device, rescues a missing crew member and escapes as La Forge seals the breach.