Armus was a malevolent entity created by an unknown race.
Armus was born as a by-product 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 black viscous liquid that could also assume a vaguely humanoid shape. Tricorder scans could not register the substance Armus was composed of. 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 it was, in fact, feeding off of the phaser energy. Captain Picard also assumed 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.
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 leaving the planet, and Vagra II was declared off-limits