Armus

From Memory Alpha, the free Star Trek reference.

Armus was a corporeal entity created by the inhabitants of Vagra II, in an attempt to shed all the evil from within themselves. However, Armus' amoral nature led him to destroy the Vagran civilization, leaving the planet barren and desert-like, forcing the Vagrans to evacuate their homeworld. This left Armus starved of his 'playthings'.

When a shuttlecraft from the USS Enterprise-D crash-landed on Vagra II in late 2364, Armus held its occupants, Deanna Troi and Ben Prieto, hostage. When the Enterprise arrived to recover its crewmembers, Armus blocked their passage, killing Tasha Yar and almost drowning Will Riker. Eventually, Captain Picard was able to distract Armus long enough to beam the shuttle occupants and himself off the surface. The shuttle was destroyed to prevent Armus' escape, and Vagra II declared off-limits.

Armus' physical form resembled black oil, and was able to control others and the environment around him. Armus fed off energy, such as phaser beams. (TNG: "Skin of Evil")

Armus was designed by Andrew Probert, played by Mart McChesney, and voiced by Ron Gans. The character might have been named for TNG writer Burton Armus. In the Special Features portion of the Season One DVD, Jonathan Frakes explains that Armus was made from Metamucil and black printers' ink.