Demonbane/Trivia

Visual Novel only

 * There is a poster for Phantom of Inferno in the cinema, during Elsa's walk around the town.
 * Music from the soundtrack of Kikokugai, another Nitroplus Visual Novel, also plays in one of the movies Kurou can watch.
 * Doctor West at one time sings the lines from opening of Vampirdzhija Vjedogonia, still another Visual Novel from the same company.
 * The second prequel novel, Assault of the War God, gives the name of Ruri Hadou's mother as Augusta Derleth, from August Derleth.
 * Nya gives Alison a Mirror of Nitocris. Soon afterward, Alison brings various beings from Alice in Wonderland to Arkham City. The mirror is also named after a short story by Brian Lumley.
 * Al names the Shoggoth Team Pet Dunsany. This is a reference to Lord Dunsany, who was a source of inspiration for Lovecraft's Dream Cycle.
 * Incidentally, the Shoggoth itself first made an appearance in At the Mountains of Madness where it rolled over and crushed giant albino penguins. Al is somehow able to procure fresh penguin meat from the South Pole for Dunsany (see Ruri's route).
 * The final battle involves a lot of shifting through time and space. Kurou visits a few of the more infamous places in the Mythos:
 * Celaeno, in the Pleiades Star Cluster. Notes taken from the Great Library form the Celaeno Fragments (as seen in The Trail of Cthulhu).
 * Yaddith, from Through the Gates of the Silver Key. Kurou mentions the protagonist Randolph Carter and his alter-ego (of sorts), the alien wizard Zkauba.
 * Yith (the black, aeon-dead orb in space) from The Shadow Out of Time.
 * The Darkness of N'kai, which appears in The Whisperer in Darkness.
 * Episode 13 of Leica's route is the aptly named Call of Cthulhu, in reference to the short story of the same name.
 * Episode 14 of Ruri's route is named after The Haunter of the Dark, another one of Lovecraft's short stories.
 * The description of Azathoth near the end (daemon sultan, flutes, nuclear chaos and so on) is taken from The Dream-Quest of Unknown Kadath.
 * The thing that attacked Kurou in the library and traumatised him against magic for life was, quite literally, The Dunwich Horror.
 * Hunting Horrors are Mythos monsters too, taken from The Dream-Quest of Unknown Kadath and are servants of Nyarlathotep.
 * Metatron and Sandalphon are angels that were said to have been humans before. In some texts they are twins.
 * The Tyrant has a monologue in Leica's route about how there is no strength left in humans to oppose the forces of the Mythos aside from Kurou and Al-Azif. (Please help me with this one, I'm no Mythos scholar)
 * "The power of the sage who defeated the Children of the Gate and Key cannot reach." -- Yog-Sothoth is also known as The Key and the Gate, and his known children include Wilbur Whateley and his brother from The Dunwich Horror. That would make the sage Professor Henry Armitage. Armitage certainly had to enlist help to kill Whateley's brother, which would make his power pretty insufficient for fighting off giant robots.
 * "The blind sage who bears the knowledge of Celaeno has been sealed somewhere." -- This is, of course, Laban Shrewsbury, the author of the Celaeno Fragments. A short time after returning from his 20-year journey escaping the minions of Cthulhu he disappeared again under mysterious circumstances.
 * "The society that stands up to the evil gods on behalf of humanity has not even been formed yet." -- This seems to be against the grain of most Mythos stories where the protagonists are either alone or connected by loose associations and temporary alliances due to circumstances, unless we're talking about Delta Green? (A shot in the dark, admittedly, but otherwise I got nothing.)
 * Timaeus and Critias are the titles of two dialogues written by Plato in which he talks about the creation of the world and its universal laws, as well as the history of Atlantis
 * In the epilogue Doctor West's Destroyer Robot is subtitled ~Everything for Big Afro~
 * Al brags that she's more persistent than the Hounds of Tindalos, which is quite an achievement because they pursue their quarry without end through the angles of time. Better find a nice round bubble-shaped room to hide in, Kurou...
 * Kurou once quotes Amuro's "You hit me...not even my own father hit me."
 * During the Beach Episode, he begs "Give me liberty or give me death!
 * Doctor West at one time goes "Listen to my song!!!" and "Here's my Love Heart!!!"

Anime only

 * Episode four, "The Invaders", is a Cthulhu Mythos story by Henry Kuttner.

Anime and Visual Novel

 * Episode/chapter one's title, "I AM PROVIDENCE", is named after a line in one of the letters HP Lovecraft had written during his life. In 1977, fans of Lovecraft created a headstone to mark his burial place and on it they put the line from his letter "I AM PROVIDENCE".
 * Miskatonic University is a fictional university in the fictional town of Arkham, Massachusetts, created by Lovecraft and used in many of his stories. The Necronomicon is also a recurring creation of Lovecraft's; in At the Mountains of Madness, several Miskatonic scholars have actually read that book of forbidden and maddening knowledge, and the University itself has a copy,
 * This becomes significant in the Visual Novel where Al is able to channel herself through time and space using the copy in the university library.
 * Kurou's name is the Japanese romanization equivalent of the name "Titus Crow", the main character in a set of horror fiction novels written by Brian Lumley.
 * Anime episode five's title (and that of Visual Novel chapter 7), "The Shadow Over Innsmouth", is a story of the same name by HP Lovecraft.
 * Episode nine / Chapter 12, "The Hunt" is a Cthulhu Mythos story by Henry Kuttner.
 * Episode eleven's title shares its name with Chapter 14 of Al's route, "The Return of the Sorcerer", which would be a nod to Clark Ashton Smith's "The Return of the Sorcerer". Clark Ashton Smith was a correspondent of Lovecraft while he lived. In the story itself, the Necronomicon is quoted.
 * The last episode/chapter "Strange Eons" is a name taken from the book written by Robert Bloch of the same name. Bloch was a good friend of Lovecraft and best known for the Live Action Adaptation he wrote called "Psycho". It references a line from the Necronomicon mentioned in several of Lovecraft's stories: "That is not dead which can eternal lie / And with strange aeons even death may die".
 * Al-Azif's name is derived from the Arabic name for the Necronomicon, "Kitab al-Azif."
 * Azrad's name is taken from the name of the author of the Necronomicon, Abdul Alhazred.
 * The Shining Trapezohedron is the name of an ancient artifact in The Haunter of the Dark, coincidentally associated with.
 * The Anticross and their Grimoires are walking shout outs as well:
 * Master Therion is a pseudonym of occultist Aleister Crowley. His mech Liber Legis also named after Crowley's book, Liber AL vel Legis (The Book of the Law). Etheldreda was the name of Aleister Crowley's dog, and her other namesake, the Pnakotic Manuscripts, first appeared in one of Lovecraft's short stories, Polaris. Incidentally, they're not joking when they say she's the first grimoire. Polaris was the first appearance of a grimoire ever in the Mythos.
 * Augustus' The Golden Bough is a treatise on theology mentioned in The Call of Cthulhu.
 * Vespasianus' Book of Eibon appears in several of Lovecraft's stories (The Haunter of the Dark, A Shadow Out of Time) while Cykranosh is the Hyperborean name (in the Mythos) for Saturn.
 * Nero's Unaussprechlichen Kulten was actually created by Robert E Howard and is Lovecraft's Gratuitous German translation for Nameless Cults, which calls forth the appropriately-named Deus Machina Nameless One. It's actually more like Unspeakable Cults, but regardless, that name works just as well.
 * Titus' Culte des Ghoules is another creation of Bloch's. Ogre has four arms, probably reference to Gug, the subterranean race of the Dreamlands.
 * Caligula's Cthaat Aquadingen is from Lumley's work.
 * Claudius' Celaeno Fragments, as mentioned above, are the work of Professor Laban Shrewsbury, whose name also gets dropped by Kurou as one of the professors at Miskatonic. Byakhees are creatures that are capable of flying through space, that appear in The Festival, by Lovecraft, and are said to be servants of Hastur (Hastur, Has-).
 * Tiberius' De Vermis Mysteriis appears in The Shambler from the Stars, a Bloch story. Beelzebuth is an allusion to the ancient god Beelzebub or the Lord of the Flies.
 * Ruri's butler Winfield is named after Winfield Scott Lovecraft, the father of the author.
 * Dig Me No Grave is a Mythos short story written by Robert E Howard.
 * Black Lodge is named after the parallel hotel in Twin Peaks.
 * Dr. West's name comes from the Lovecraft story Herbert West -- Reanimator.

The Sequel

 * Laban Shrewsbury is one of playable character piloting Deus Machina Ambrose, obviously reference to Ambrose Bierce, pre-Lovecraft author who created the name Hastur. Him being playable character might be reflection of how Hastur is rather benevolent god in Bierce's Haita The Shepherd.
 * Another Blood is Necronomicon: Blood Edition, this might be referrence to Necronomicon Ex-Mortis. Note that this book isn't relate to Al Azif, and thus Another Blood come from alternate future that shouldn't exist.
 * As the Final Boss, Nyarlathotep appear as Clockwork Phantom, which is eleborate version of Tick-Tock Man from mythos.