Cytherian
The Cytherians are a super race who dwell in a star system near the core of the Milky Way galaxy. The Cytherians do not like to travel out of their home system, so they send probes out into space to contact other civilizations and bring visitors back to their own system for an information exchange.
They appear only in the TNG episode "The Nth Degree", making them one-episode aliens, as well.
Technology
The Cytherian probes have technology for altering electronic or biological systems they encounter, upgrading them to make a journey to the Cytherian homeworld possible and implanting an irresistable imperative to complete the journey. The Cytherian technology is not compatible with all alien technologies that it encounters; it was not able to modify Federation computers based on isolinear chips to have the necessary computing capacity and programming. It was, however, able to alter the brain of Reginald Barclay to accomplish its task.
Cytherian probes are highly resistant to phaser fire, but vulnerable to photon torpedos.
Threat Assessment
Almost nothing is known about Cytherian military technology, but their interstellar travel technology gives them the means to send military assets across the galaxy in seconds, and their advanced computer and propulsion technologies suggest that they would also have advanced weapon capabilities. While potentially quite dangerous, they are unlikely to react to any military force that does not directly threaten their homeworld.