"Laws!? Logic!? You humans credit yourselves far too much, while simultaneously limiting yourself to the preconceived notions of this world... Your motives, your emotions, your actions... Ufufu, do you ever wonder if any of them is even real..?"
The Demon has Employed All His Energies to Ensnare My Judgment
Descartes' Evil Demon
Type: Conceptual/"Anti-Reality"
Rank: EX
Range: Varies
Max Targets: Varies
Berserker's true nature; a sentient reality marble embodying his concept. It is a constantly active Noble Phantasm, despite normally being limited by humanity's collective (lack of) belief in his nature. In its restrained state, this Noble Phantasm grants Berserker a very limited capability of rejecting the fundamental laws of the world, normally allowing him to teleport in small distances, perform minor telekinesis, levitation, illusionism, and cause beams and projectiles to curve. These capabilities stem from claims that in order to deceive the human senses and make them question their perception of reality, the so-called Evil Demon must possess some form of omnipotence; the capability to subvert or even shatter the perceived fundamental laws of reality. That said, due to Berserker's metaphysical nature of existence, he is incapable of directly inflicting harm to his targets. At most, he could use his universal law-defying powers to attack them with their own projectiles or other objects in the vicinity.
...Of course, that's far from the true power of this Noble Phantasm. If the former simply represents "what an omnipotent, metaphysical demon could hypothetically do within the constraints of human comprehension", the true form of this Noble Phantasm takes it up to eleven and taps into the reality-redefining powers that such an existence would be capable of should they truly exist.
Berserker can only activate this Noble Phantasm once a majority (50% + 1) of the collective human unconscious believes and fully accepts his existence and all its implications. Upon its activation, Berserker isolates a portion of his surroundings akin to a Reality Marble of a bounded field. Yet unlike those examples, where the isolated field must still follow the fundamentals of logic, Berserker's field effectively qualifies as its own "world" where Berserker himself may freely rewrite its laws and redefine its "reality". As this "isolated world" no longer follows the laws of the known world, Berserker himself may directly interact with its inhabitants, increasing his offensive capabilities. The range of this "isolated world" varies depending in the number of individuals who believe and accepts Berserker's existence, ranging from a small town at minimum to an entire world at most (althought it's highly unlikely that the majority of humans could accept his existence to this extent). Of course, the mana cost to sustain this world is enormous due to both Gaia and Alaya's corrective forces trying to shut it down at all costs. At its minimum range, a Master with regular magic circuits would be unable to support it for more than a few minutes without dying, or risking permanent damage to their circuits.
There are two specific ways that a Servant could shatter Berserker's personal reality and remain relatively unscathed:
The first, of course, is to use an Anti-World Noble Phantasm there. Upon impact, that world would shatter, and by extension, since Berserker possesses a more tangible body within that reality, it may also be possible to kill him in his own reality and emerge safely back to our reality.
The second way to survive this Noble Phantasm would be to possess an insanely strong denial against the concept that Berserker embodies:
Someone who endlessly, insanely seeks an inherent meaning to life, even when there is none...
Someone who believes so strongly in their own ego that the notion that they are not in control could never cross their mind, even for a split second...
Someone who is so deluded in their own definition of "reality" that they could never imagine the prospect of "reality" that is even slightly different than their own definition of it...
Should any of these individuals enter Berserker's reality, that reality would effectively crash due to their sheer denial, potentially even inflicting considerable damage (if not killing him outright). Ultimately, only someone with an utterly distorted view of "reality" can utilize this weakness.