A Cytherian probe was a device built by the Cytherians to seek out other species and bring them to their homeworld. The probe was able to move at impulse and warp velocities without emitting particles or generating a subspace field, indicating its propulsion technology to be utterly alien to the Federation. It was highly resistant to phasers but not to photon torpedoes.
In 2367, a Cytherian probe unsuccessfully attempted to contact the Argus Array, disabling the Array's computer in the process. When the USS Enterprise-D arrived and sent its shuttle Feynman to investigate, the probe attempted to contact the shuttle computer and its occupants, Geordi La Forge and Reginald Barclay. La Forge was protected by his VISOR, while Barclay was affected. The probe then pursued the Enterprise, necessitating its destruction by a spread of photon torpedoes.
After having his brain reprogrammed by the probe, Barclay exhibited greatly increased intelligence and a compulsion to bring the Enterprise to the Cytherian homeworld, thus fulfilling the probe's mission. (TNG: "The Nth Degree")