Nightmare is a character created by BCLEGENDS, and the main antagonist in FAW V: The Nightmare Begins, as well as a fairly important antagonist in the following FAW games, making at least one appearance in each. He is similar in physical appearance to Unown Entity, aside from the fact that he has no mouth and that his pupils are four-pointed stars, and wields immense power, able to perform practically any act within the laws of logic. His main goal is to find a way to leave the Universe itself, thereby being able to bypass logic (due to the lack of Universal laws applying outside of any of them) and thus becoming a true god; he was purportedly able to bypass logic in a pocket dimension of his, but such an act was not shown before it collapsed upon his eventual defeat, and whether he would have been able to use these logic bending skills upon other dimensions whilst in his dimension is unknown.
Nightmare was born after a small section of Unown Entity's body was separated from him during a period of unconsciousness, then stored in a jar in a wall for eight years or so. He was released by a human scientist and tipped onto a powerful ruby owned by the company the scientist worked for, resulting in the somewhat brutal death of said scientist, as well as, incidentally, the creation of the light and shadow stones. He then waited in a form of limbo with a machine of his (of unknown origins, but perhaps formed from rearranged atoms that were in the murdered scientist's body) for more than half a century, at some point taking control of the remains of the Shadow Core's destroyed pocket dimension and continuing his wait there, until given the perfect opportunity to escape when Unown used a Hammerspace device upon himself that would usually place users in a half-dimensional state.
Upon the takeover of Unown's body, Nightmare disguised himself as Unown, then proceeded to take pieces from the monsters that the team destroyed over the course of FAW IV: The Curse and the first part of FAW V: The Nightmare Begins and store them in his pocket dimension, to later use as fuel for his machine. He was eventually found out by Jaye, but by then he already had enough fuel for the machine, and successfully powered himself up to god-like levels. He proceeded to transport the whole team to his dimension, fighting them for a time, until Unown returned and helped them fight Nightmare, at which point it was revealed that Nightmare would slowly weaken in the presence of his "predecessor". He eventually forced the team out of the pocket dimension and imprisoned the Shadleans in order to torture them. Whilst torturing Charlotte, however, she snapped and transformed into A-C.HAR.L. She was promptly forced to remain still, at which point the team were brought back into Nightmare's dimension and she was used as a weapon against them until the reallocation and defeat of another threat, the Para-Site.
Nightmare continued to fight the team (as well as Joseph and an anime girl who referred to herself as Denbo, but was dubbed Annie May, or Anne for short, by Unown and the other Shadleans), summoning up a giant octopian creature which he named Nsarglyde, as well as regularly switching between forms, which included a Liquid Form, a Plasma Form, a Solid Form (essentially what Nightmare would be like as a human), an Energy Form, and finally an Interdimensional Form, at which point he managed to effortlessly slaughter two of the team, Chris Crippler and Zackary Seld, via a dimensional rip around his form. The team escaped at this point in order to regroup, and Nightmare formed a huge castle with various traps and such, culminating in a maze of lasers formed out of strange matter. However, virtually all of these were bypassed when the rooms were rearranged by Martin Carpen, and the maze itself was flown through by a shrunken Anne/Denbo. A few minutes before, Unown had gone through a different set of rooms to the others, then appeared out of a ventilation system right next to Nightmare, depowering him to his regular form. Anne/Denbo, who currently inhabited the body of Catherine, proceeded to kiss Unown, empowering him with the emotions of the other team members, the result of which was a giant pink beam fired by Unown at Nightmare. This successfully damaged Nightmare, and badly enough so that he had a form of superpower meltdown, destroying the castle in the process. At this point, he was severely weakened, but he still had enough power to infest Unown's mind. He did not, however, have enough power to overpower it, and promptly became trapped within. As he was so severely weakened, as well as seemingly cut off from the rest of the world, the collapse of Nightmare's pocket dimension was triggered, although everyone in there escaped via portals.
Nightmare remained trapped within Unown's mind, becoming drained of his original powers in the process, until around halfway through FAW VI: The Last, at which point Unown was hit by a radiation blast fired by Dermot. Nightmare finally escaped his mental prison, returning to full power in the process, and went on another rampage, this one much shorter than the last. After Unown was once again split from Nightmare, the latter was taken to a region in space by Martin, who currently possessed a body formed out of experimental, portal-esque particles called holons. After powering himself up to his Interdimensional Form, the two dueled each other, culminating in Nightmare teleporting away seconds before forming a black hole around Martin, to little effect. He then met up with an apparently plot-empowered girl called Gwendolyne, who agreed to join him in his quest, after some deliberation on both sides. Still powered up, Nightmare then formed a fortress in space and stole small sections of the bodies of Charlotte, Fred, Catherine and Alex, and used his power to turn these into copies similar to him, forming Reaper, Starstorm, Pathos and Eclipse, but draining himself severely in the process. These five now formed the Aeonica, and presumably began plotting to destroy the FAW group.
Partway through FAW VII: The Eye Of The Storm, some five thousand years later, a dimensional rift was formed during a battle with Paula. This rift apparently led to the Aeonica's fortress, or at least somewhere nearby; in either case, Nightmare returned once again, this time with four beings of a similar power level to his, as well as Gwendolyne, who now had the ability to form her body into any substance she desired thanks to the Aeonica. Another short, bloody battle was fought, this time involving not just the FAW team, but a group of Hammerspace legends called together by Martin as well. The two groups eventually escaped to Hammerspace, moment before a giant explosion obliterated most of the surrounding area of the battle. Nightmare and most of the Aeonica were alright, but Pathos' original body had been destroyed, and her mind was now trapped within that of Maz's, another telepath. With an air of what could almost be described as dejection, the remaining Aeonica and Gwendolyne returned to the space fortress, and Nightmare has not been seen since.
His powers before the activation of his machine were the same as a regular Shadlean's, but also included the ability to phase between dimensions, the ability to see different kinds of radiation along the spectrum and telepathy similar to Catherine's. (Incidentally, a form of mass telepathy is how Nightmare communicates when not disguised or in his Solid Form, although this can be focused to one person if necessary.) Being what is essentially a powered-up evil clone of Unown, he probably had the ability to slow time as well, but such a power was not shown until after his empowerment, at which point he could do anything he liked anyway. The ability to see in differing forms of radiation was transferred to Unown during Nightmare's imprisonment in his mind, but was taken away after Nightmare escaped.
Upon his first appearance, Nightmare was shown to slowly become weaker and weaker in the presence of Unown, perhaps due to the latter being the former's predecessor, and thus draining energy back from Nightmare. However, Nightmare could hold this off by himself draining energy from attacks thrown at him, and he seems to have become immune to Unown's presence after escaping, and perhaps due to, his imprisonment in the latter's mind. Also, his only proper defeat came after an emotional blast was fired at him, but whether such an attack would work again is debatable.