Armus

Armus was a malevolent entity created by an unknown race.

Origins
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 spred 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. 

Attributes
Armus was physically a black oil-like 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 impervious to phaser fire, and Data theorized it was, in fact, feeding off of the phaser energy. Captain Picard also assumed Armus was immortal though this hypothesis was never confirmed.

Armus was perpetually in a state of undirected and unfocused rage. However, when that rage was focused it could generate a powerful forcefield around itself and part of the surrounding area. 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 crashland on the planet in 2364. 

First Contact
First contact with Armus occured in 2364 after the crash of carrying Counselor Deanna Troi. The USS Enterprise-D raced to Vagra II in the Zed Lapis sector to rescue the shuttle and beamed down an away team. Enroute to the crash site the away team encountered a mysterious inky black fluid with the consistency of road tar. When they tried to get around it, the strange pool blocked their path.

Not long after it was assumed that the pool was in fact some sort of lifeform, Armus introduced itself. During this first encounter it generated a field that would kill Enterprise-D security chief Lieutenant Tasha Yar. After killing Yar, Armus believed it would feel amusement and that the rescue effort would be abandoned. This was not the case, however. Armus then realized torturing the shuttlecraft survivors 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 enough to allow beaming the shuttle occupants and himself off the surface. The shuttle wreckage was destroyed with a photon torpedo to prevent any possibility of Armus leaving the planet, and Vagra II was declared quarantined. 

Escape from Vagra II
After being marooned on Vagra II once more, Armus' rage intesified and was now focused towards the Federation. Vainly it sought a way to seek revenge on the entities that marooned it there. Its chance for vengeance came in 2376 when a lone Dusk Nobody found its way onto Vagra II.

The Dusk attempted to absorb Armus' heart, but the darkness inside Armus proved overwhelming and Armus absorbed the Dusk instead. In doing so, Armus gained the ability to jump from dimension to dimension. Its first contact with the Nobodies and the Heartless lead it to raise an army of the dark creatures to reap vengeance.

Armus' first dimensional jump lead it to an alternate version of Earth circa the year 1985 in a place called Hill Valley. In doing so it would encounter a man named Biff Tannen, the CEO of a waste disposal company called BiffCo. Appealing to Tannen's desire for power and wealth and playing off his fears, Armus influenced Tannen to be his right hand in seeking and uniting an army of Heartless to unleash upon the galaxy.

Armus would first encounter Federation personnel from an away team of the USS Voyager (NCC 74656) when the Delta Flyer was sucked into a spatial rift into one of the dimensions it visited. (Star Trek: Voyager - Dark Realm)