A solanogen-based lifeform
The solanogen-based lifeform was a humanoid species whose molecular structure is based on solanogen, a substance that is only stable in subspace. They originated from a tertiary subspace manifold, and could not survive unassisted in normal space. Physically, they were characterized by bulbous eyes, beaks, and clawed, three-fingered hands. Most of their bodies were hidden by hooded metallic robes. Their language consisted of a series of rapid clicks.
In 2369, the lifeforms were accidentally drawn to the USS Enterprise-D when Geordi La Forge enhanced the sensors by transferring warp power to the deflector array, thus sending a signal deep into subspace. They accessed the Enterprise by creating a spatial rupture using spatially inverted tetryon particles, and created a pocket of this universe in their realm in order to study the Enterprise crew.
Over a number of days, the lifeforms secretly abducted numerous Enterprise crew members and performed experiments on them. Their subjects included William T. Riker, whose arm was amputated and then reattached, and Edward Hagler, whose blood was transformed into a liquid polymer. Other crew members abducted included La Forge, Data, Worf, Kaminer, and Sariel Rager. Their subjects retained little conscious memory of the experiments, though they were able to reconstruct some parts of the experience with the help of the holodeck.
It was believed that the lifeforms were attempting to create a pocket of their native domain in this universe, which caused the spatial rupture to expand and threaten the Enterprise. By giving a homing beacon to Commander Riker, the crew was able to localize the lifeforms' specific origin and seal the rupture with a coherent graviton pulse. Shortly before the rupture was sealed, the lifeforms sent a probe of some kind through, which disappeared into deep space. All Federation starships were subsequently warned against the sensor modifications that originally attracted the lifeforms. (TNG: "Schisms")