El-Adrel IV lifeform

The El-Adrel IV lifeform was an extremely aggressive, vaguely humanoid entity native to the planet El-Adrel IV. It generated highly localized electromagnetic fields.

In 2368, Starfleet Captain Jean-Luc Picard and Tamarian officer Dathon were transported to the surface of El-Adrel IV by the Tamarians to defeat the entity, in the hopes that doing so would establish communications between the two species. The Tamarian language was based upon metaphors in their history and so any outsiders would be unable to learn it without knowing the Tamarian history. By re-enacting the legendary story of Darmok and Jalad against "the Beast" at Tanagra - Picard and Dathon against the El-Adrel IV lifeform - the two were able to successfully communicate with one and other.

However, this came at a price; the El-Adrel lifeform struck a mortal blow to Dathon, though his death was not in vain as it opened up a new era of communication between the Tamarians and the Federation. 

Background information
The El-Adrel IV lifeform was portrayed by three-time stunt actor Rex Pierson, who received no on-screen credit for his appearance.

The scenes featuring the El-Adrel lifeform were filmed on location at Bronson Canyon, in an area below the famous Hollywood sign, on and, under the supervision of stunt coordinator Dennis Madalone. (Star Trek: The Next Generation Companion, p.177)

To create the creature, the makeup department built a huge head, first by creating a mold out of two hundred pounds of plaster and then by casting it out of polyurethane in three separate pieces. Also built by the makeup department was the creature's hands, while the costume department created the body. In the monster segments, the creature was assembled using computer-generated optical effects that put the shots of the head, body and hands together and enabled the creature to move and twist its head. For the closeups, the head was mounted on a stand and fitted with a lever that opened and closed its mouth. (Star Trek: Aliens & Artifacts, P.121)

The creature's unusual "glowing" was realized with a less expensive version of the -type melting effects. Going from tape to film and back again, the creature was shot against a blue screen on fast video and developed on film 10 stops over the exposure. (Star Trek: The Next Generation Companion, p.177)

The mask worn by Pierson was sold off on the It's A Wrap! sale and auction on eBay.