Armus was born as a by-product or personification of a procedure in which a race of "Titans" brought out from within themselves all evil and negative attributes that had bound them to destructiveness. The unwanted substance spread and coalesced into a dank and vile second skin. The race rejected this "skin of evil" and abandoned it on the barren planet Vagra II in the Zed Lapis sector.
Armus manifested as a slick of black viscous liquid that could also assume a vaguely humanoid shape. Tricorder scans could not register the substance Armus was composed of, but rather could determine that it lacked a neural and circulatory systems, internal organs, nor recognizable proteins, and no known cellular structure. Furthermore, and observing its movements, its locomotion could not be determined, as it also appeared to lack skeletal framework or musculature. Though showing no sign of intelligence, much less a brain, showed evidence of thought.
Armus was also capable of enveloping humanoids and incorporating them into the liquid, where they would remain conscious while in liquid form. Armus was capable of then inflicting suffering upon its victims while they were in this state. Armus was impervious to phaser fire, and Data theorized that it could, in fact, feed off of phaser energy. Captain Picard assumed that Armus was immortal.
After the initial abandonment, Armus was left in a state of undirected rage. However, when the rage was focused, Armus was capable of generating, around itself, an intense undefined force field. With this energy field, it was capable of blocking sensor scans, communications and transporters. It was also capable of using psychokinesis and teleportation on at least humanoid-size organisms within the field. Armus was also capable of inflicting energy discharges that caused synaptic damage to humanoids, killing them. Presumably, this field was also responsible for causing the nearby Shuttlecraft 13 to experience a massive systems breakdown and crash-land on the planet in 2364. He is also an Empath, which was demonstrated when speaking to Deanna Troi about how he could feel the worry and pain he was inflicting on her fellow crew members and how he got enjoyment out of it.
Armus waited for someone to arrive for the rescue of the two shuttlecraft passengers, in order to sadistically torture the rescuers. After killing Lieutenant Natasha Yar with an energy discharge when she tried to walk to the downed shuttle, Armus expected to feel amusement. It also expected that the USS Enterprise-D crew would abandon the rescue effort. Even after they didn't, Armus realized torturing them would never be amusement enough. It wanted passage away from Vagra II, to find the beings that initially abandoned it there. Eventually, Picard was able to distract Armus long enough for the energy field to weaken to a point whereby it allowed beaming the shuttle occupants and Picard off the surface. Following the beam-up, the shuttle wreckage was destroyed with a photon torpedo to prevent any possibility of Armus using it, and Vagra II was declared off-limits. (TNG: "Skin of Evil")
In an alternate timeline, Lieutenant Yar was not killed by Armus, and was still the security chief on the battleship USS Enterprise-D in 2366. Guinan, who somehow knew of Tasha's other fate, described it as a "senseless death in the other timeline." (TNG: "Yesterday's Enterprise")
Several years after Yar's death, the Enterprise visited Yar's homeworld, Turkana IV. Here Data met and mentioned to Tasha's sister, Ishara, that her sibling was killed by a malevolent entity. He further explained that "[s]he was killed as a demonstration of the creature's power, without provocation." (TNG: "Legacy")
Concept art for Armus was designed by Andrew Probert. (The Art of Star Trek, p.102) The costume creation was divided between Michael Westmore's department (to design the head piece of the suit) and Makeup & Effects Laboratories (to create the body). (Star Trek: The Magazine Volume 2, Issue 12, p. 26) The head piece was made out of soft polyfoam with a latex skin over it. It was sculpted by Westmore and Gerald Quist. (Star Trek: The Next Generation Makeup FX Journal) Westmore sculpted the head piece around a cast of the actor's head which he had taken. He also embedded goggles into the prosthetic piece, to prevent the black slime from rushing into the actor's eyes. (Star Trek: The Magazine Volume 2, Issue 12, p. 26) There were three mouth holes on the head piece, designed in such a way that fingers could be easily inserted to clear the actor's mouth from the black goo. (Star Trek: The Next Generation Makeup FX Journal)
The goo was made from Metamucil and black printers' ink. (TNG Season 1 DVD special features) According to Michael Westmore, the goo was water-soluble methocel material and the ink was water-based. (Star Trek: The Next Generation Makeup FX Journal)
Armus was played by Mart McChesney and voiced by Ron Gans. Because no oxygen tank was provided to McChesney, the scenes of Armus submerging and rising from the pit of black goo were timed with a stopwatch. (Star Trek: The Next Generation Makeup FX Journal) However, the body portion of the costume disintegrated during filming, after reacting with the goo, each day of shooting. Several more costumes were ordered from the manufacturer after the first of two original ones fell apart. Michael Westmore has been unable to find an explanation for the disintegrations, as the goo was supposed to be inert. (Star Trek: The Next Generation Makeup FX Journal) "I don't know what else was in it," admitted Westmore, "because it caused the glue in the costume – very strong shoe glue – to undo itself, and the costume would fall apart." (Star Trek: The Magazine Volume 2, Issue 12, p. 26)