The Cytherian probe was an alien device found near the Argus Array in 2367, which reprogrammed Lieutenant Barclay's mind so that he would bring the Enterprise-D to the Cytherians.
The probe appears in the Next Generation episode The Nth Degree. The Enterprise is sent to repair the Argus Array, a huge subspace telescope that has stopped working, and finds an unknown object beside it. La Forge and Barclay examine it from a shuttlecraft, and a burst of light from the probe knocks Barclay unconscious.
The probe then follows the Enterprise, giving off rising radiation and matching every attempt to outrun it. Barclay, who is rapidly becoming far more intelligent and confident, finds a way to boost the shields so that a torpedo can destroy it.
Its work is already done. Barclay links his mind to the ship's computer and takes the Enterprise 30,000 light years to meet the Cytherians, who explore by bringing others to them.