An energy being from a cloud near Beta Renna that leapt into the Enterprise, moved from host to host and finally carried Picard away as pure energy.
The entity was a noncorporeal being living within an energy cloud the Enterprise passed through. It could travel through ship systems and inhabit people, taking control of their actions.
In Lonely Among Us, it jumped first into Worf, then into Beverly Crusher, and was involved in the death of an engineer before taking over Picard. Wanting to return home, it led Picard to beam into the cloud as energy. Picard soon wanted to come back, and the crew recovered him by using his stored transporter pattern.