Nyarlathotep was created by H.P. Lovecraft and is a fictional character in the Cthulhu Mythos featured as an Outer God. He is also symbolic of the destructive potential of Carl Jung's collective unconscious and actively seeks to agitate our universe into madness, woe, and discord.
Nyarlathotep's original character design, first seen in Shin Megami Tensei II, is actually a composite character based on several different monsters from Lovecraft's writings. Its dark, featureless appearance, with three eyes and bat wings resemble The Haunter of the Dark, one of Nyarlathotep's "1000 Masks". The pincer-like hands and spiny crown might reference the Mi-Go: crablike, winged fungoid aliens with a mass of feelers in place of a head first appearing in The Whisperer In Darkness. His black, winged body makes him looking like the Night Gaunts which appear in The Dream-Quest of Unknown Kadath.
Nyarlathotep appears as a special field boss in Nakano. During a Full Moon, if the player is holding a Shining Trapezohedron and approaches the tree at the southern central area, a Fiery Three-Eyed Nyarlathotep boss will appear along with several Nightmare Black Oozes. Should Nyarlathotep and the Black Oozes be defeated before the Full Moon ends, there is a rare chance that a second, more powerful Crawling Chaos Nyarlathotep will appear to take their place, who is accompanied with several other, smaller Crawling Chaos Nyarlathotep. This boss will drop a Crown Shard when defeated, which can be traded with the Junk Collector in Souhonzan for Nyarlathotep's plug-in, Crawling Confusion.
He is given a prominent plot role, known as The Crawling Chaos, as the counterbalance to Philemon in the Persona series. Unlike Philemon, Nyarlathotep is able to operate in the outside world and take an active role. Throughout the series he employs the use of Shadow Selves of characters to accomplish his goals. Shadow versions of Tatsuya Suou, Lisa Silverman, Eikichi Mishina, Maya Amano and Yukino Mayuzumi appear as bosses in Innocent Sin thanks to rumors. In Eternal Punishment, Maya Amano, Katsuya Suou, Ulala Serizawa, and Baofu all have Shadows Selves which attack the party as bosses in pairs. Nyarlathotep is also capable of appearing as anyone he chooses. It is revealed in Eternal Punishment that he is represented by a black butterfly.
Nyarlathotep is the persona of Takahisa Kandori. After Kandori is defeated by the protagonist and his friends, Nyarlathotep forcibly enters Kandori's body and transforms him into God Kandori, saying he will grant him the power he seeks. Retroactively, Nyarlathotep has been mentioned to have engineered the whole incident with the intention of giving the humans a chance to destroy all of reality, allowing him to win his bet with Philemon.
An alternate form of Nyarlathotep can be fused in the Velvet Room by adding the "Faceless Mask" totem, dropped by God Kandori, to a fusion with a Tower Arcana Persona result. Nyarlathotep has "Worst" affinity with every party member except for Reiji Kido, Kandori's half-brother.
Both he and Philemon are acting on a bet to see if humanity will become enlightened or ultimately destroy itself. Nyarlathotep decides to push things in his favor by helping humanity end. He appears as Akinari Kashihara, Jun Kurosu's late father, and the Fuhrer, whom was supposedly revived by rumors.
He serves as the game's final boss in the form of the Great Father (グレートファーザー, Gurēto Fāzā)?, a fusion of the main cast's fathers. The Great Father's arms and legs are each a particular character's father; Lisa Silverman's father is the right arm, Maya Amano's is the left arm, Eikichi Mishina's is the right leg, and Tatsuya Suou's is the left leg. The head takes the shape of Akinari. It is worth nothing though, the "head" (Akinari) uses fire spell (Tatsuya's signature element), while the "left leg" (Tatsuya's father) uses wind spell (Jun's signature element) and calls Jun's name while attacking. It is unclear if this is an error or intentional as the limb would otherwise be unable to call out to Tatsuya due to the fact that the player can change his name.
He is voiced by Jin Yamanoi (Japanese; for both versions of Innocent Sin and Eternal Punishment) and Tom Wyner (English) in Eternal Punishment. Patrick Seitz, however, voices him in English in the PSP version of Innocent Sin.
Nyarlathotep fulfills the same role as he did in Innocent Sin. While he does not restore Tatsuya Sudou's memories of the events from the previous game, he does talk to him and tease him with comments intended to spark the general idea of the same events in Sudou's addled mind, so he would be driven to repeat them in the Other Side. He appears before the party as Tatsuya Suou. It is revealed later that the Time Count is Nyarlathotep in disguise.
Nyarlathotep is described as the destructive side of Humanity's collective unconscious, and as such, the living embodiment of Mankind's collective evil; every thought, action or feeling that derives in experiencing the desire to bring harm to others or oneself in any form fuels Nyarlathotep's existence. His reason for providing the human race with the means to drive itself to destruction is given as a bet between him and Philemon, who holds that they instead will ascend to a greater destiny.
Because of his past defeat in Innocent Sin, he took the opening Tatsuya Suou provided him with when he refused to abandon his memories of the Other Side, implementing a new plan to let Mankind destroy itself, and branded Tatsuya with a mark to symbolise the doom that would befall Humanity should either Maya Amano, Eikichi Mishina, Lisa Silverman, or Jun Kurosu regain their memories of the events of Innocent Sin, and delighted in tormenting him as Tatsuya Sudou attempted to force the restoration of the hidden memories with a terror campaign similar to the one he had followed on the Other Side.
In the end, a new group, formed by Tatsuya, Maya, Baofu, Tatsuya's brother Katsuya, and Ulala Serizawa again stood against him and his plans of extermination. However, following his second defeat, he was banished to parts unknown. He wasn't destroyed forever, though, because he is an undeniable, indelible presence in Humanity's heart, and may still return.
In Tatsuya's new scenario in the remake, Shiori Miyashiro's self is shattered into Trapezohedrons; Nyarlathotep's avatar, The Faceless God, seizes the last of the eleven stones, the one containing Shiori's shadow, and confronted Tatsuya and his group in the Kadath Mandala. Upon his defeat, he tells the party that soon they'll meet the real him in the flesh and is forced to leave behind not only Shiori's Trapezohedron, but also Randolph Carter's Trapezohedron of Emotion.
"A god of the Cthulhu mythos. He came to Earth in ancient times from outer space and is known as the Crawling Chaos. He acts as a proxy to his masters such as Azathoth, who have no intelligence to make their desires manifest in our world. Because he can assume various forms, he is known as a faceless god. He is particularly known as the Haunter of the Dark, and he seems to have other guises on Earth as well."
—Devil Survivor Overclocked Compendium
Known as Nyalatotep. In the 8th day, God vs Hero, Nyarlathotep is one of Naoya's demons when he appears to help during the final battle against Okuninushi.
Nyarlathotep in Persona 2 duology has a musical theme titled False Kashihara (偽橿原) which shares the same motif with Philemon's. The arrangement uses eerie pads, stronger tom drum and human scream sound effect with strong reverberation.
The final battle theme against Nyarlathotep in both Innocent Sin and Eternal Punishment continues to develop the same motif which shares the same choir phrases in the opening movie music and Order of the Holy Lance battle theme from the Innocent Sin OST.
Shadow Aigis' intro for Persona 4 Arena Ultimax shows her with a black butterfly, Nyarlathotep's symbol. Despite this, he has no role in the story.
The broken Buddha statue on Kandori's Persona-Nyarlathotep in Eternal Punishment is identical to the intact Buddha face that appears as the body of God Kandori in Megami Ibunroku Persona.
The fact that Nyarlathotep is the only known character to disguise as a Persona deliberately to manipulate a main antagonist (Kandori in the first Persona and Jun Kurosu / Joker in Innocent Sin) references his roles in the Cthulhu Mythos universe; while the other Mythos gods are often described as mindless or unfathomable, rather than truly malevolent, Nyarlathotep delights in cruelty, is deceptive and manipulative - deceiving the antagonists into believing he could grant them power (which he did at the cost of nearly losing their humanity) and manipulating them into becoming his pawns - and even cultivates followers and uses propaganda to achieve his goals - using rumors, Wang Long fortune telling, Sumaru TV (broadcasting Wang Long), and the Masked Circle (Innocent Sin) / Tenchu Army (Eternal Punishment) to ultimately destroy the world around Sumaru City.