The El-Adrel IV lifeform.
The El-Adrel IV lifeform was an extremely aggressive, vaguely humanoid entity (possibly) native to El-Adrel IV that generated highly localized electromagnetic fields.
Captains Jean-Luc Picard and Dathon were transported to the surface of El-Adrel IV by the Tamarians to defeat the entity, hoping to re-enact the legendary events involving Darmok and Jalad against "the Beast" at Tanagra in an effort to establish communications between the Federation and the Tamarians. The entity mortally wounded Dathon, but not before he successfully established communications with Captain Picard. (TNG: "Darmok")
The lifeform was played by uncredited stunt actor Rex Pierson.