An unidentified and uncommunicative species whose powerful ship repeatedly attacked and boarded Enterprise NX-01 in 2151, driving Jonathan Archer's crew to finish installing their phase cannons.
These aliens are never named and never speak. In the episode Silent Enemy, their ship approaches Enterprise several times, ignores every hail, scans the vessel and then opens fire, each attack more damaging than the last.
In one encounter two of them board Enterprise. Tall, thin, pale figures, they stun and examine crew members before leaving, and their shielded ship shrugs off the plasma cannons. They finally send back a doctored recording of Archer's own voice telling the crew to prepare to surrender their ship.
Archer at first turns for Earth to have the phase cannons fitted at Jupiter Station, but Malcolm Reed and Trip Tucker build and install them on the way. An unplanned power surge makes the new weapons far stronger than expected, and the damaged alien ship withdraws. Their motives are never learned.