The Evil God of the Abyss is implied to be Cthulhu of the Cthulhu Mythos. He is an evil god of an ancient time sometime before humanity's origin. The Evil God is said to be related to the sea and octopi. He is trapped within the Sunken Spiral Castle of the Abyss.
Books containing forbidden knowledge relating to him can be found in the history of men. The original manuscript was written in a language other than that of humanity's languages (Suggested to be R'lyeh Text). The book was translated in classical Chinese during the Xia Dynasty in China, François Prelati would later translate the Chinese into Italian and create the Prelati's Spellbook. An alternate telling of the grimoire's origin describes Prelati opening a gateway to the Sunken Spiral Castle - having mistaken the Evil God of the Abyss for Beelzebub - and sealing the link into a blank Bible, turning it into a Mystic Code capable of summoning Horrors.
Katsushika Hokusai was known to gather vast quantities of reference materials to further enhance his artistic skill, he got ahold of a Chinese copy of the thaumaturgical grimoire from the Tang dynasty of China. Hokusai's images of Sea Demons still survived in the modern day.
After the "Salomon" singularity, Raum believes humanity's salvation cannot be found within this universe, and it needs to come from the outer universe, so he attempts to bring forth an Outer God to connect a path which Raum will use to bring in Foreigners from the void. Raum's first attempt at his plans was to summon the Evil God of the Abyss, but it ended as a failure. Raum opened a hole in the laws of the world allowing the Evil God on the other side of the abyss to peer in. The Evil God of the Abyss sought hosts to manifest and decided to use Katsushika Hokusai for this purpose (Yorishiro).
Hokusai, on the verge of death, was called by the Evil God of the Abyss unto the waters of the Deep. The Gentleman manifested, warning him not to go any further as it was the call of the demon of the Deep. However, Hokusai was excited to see what a demon looks like and sunk deeper. After Hokusai saw the Evil God of the Abyss, he found nothing special about the God, having never had any intention of becoming a host for the Evil God. Hokusai, as an artist, prides himself in reflecting the world about him within his art. The Evil God, by comparison, forces the world to conform to its art and is, therefore, no true artist. Hokusai declared thus that they are inherently incompatible.
Hokusai Trial QuestEdit
Ritsuka Fujimaru and Suzuka Gozen ended up in a dream world in Edo, they encountered Katsushika Oui, who is looking for her father, Katsushika Hokusai. It was originally thought that the dream belongs to Hokusai. But it was revealed to be Ritsuka's dream and the Evil God of the Abyss desires to descend unto the Servants of Chaldea and to use Ritsuka's dream to access a pathway to the real world.
Utagawa Toyokuni (Da Vinci) under the influence of the Evil God states that Hokusai has nearly reached the completion stage of Yorishiro and that Hokusai's daughter, Oui is the final obstacle as she is the only thing holding Hokusai back from reaching his desires. The allegation pains Oui, and as the dream is consumed by the taint of the Evil God, Ritsuka starts getting corrupted by the dream too. Oui's ego begins to fade into the certainty that her father has once again cast her aside. Both Suzuka and Ritsuka call her and in that moment, Hokusai is called to awaken, taking ahold of his daughter's flesh. Hokusai possesses Oui and helps to resist this, saying that he never had any intention of becoming a host for the Evil God and that he saw its face right before he died and can now draw it.
The Evil God has moved on to another attempt, which seems like it may not be the future for Chaldea.
In Fate/Zero, Gilles de Rais chants "Cthulhu fhtagn" to summon a Sea Demon from the Sunken Spiral Castle using Prelati's Spellbook; and later chants "Ph'nglui mglw'nafh wgah'nagl fhtagn" to summon the Gigantic Horror, a colossal eldritch monster stated to be a manifestation or incarnation of the evil god that rules over the extradimensional ocean connected to the grimoire. The Gigantic Horror attempts to make landfall and devour the populace of Fuyuki City, withstanding an assault from multiple Servants at once, but is destroyed by Saber 's Excalibur.
Creation and ConceptionEdit
The Evil God of the Abyss is likely adapted from Cthulhu of the Lovecraftian Mythos. This is interesting considering in the Mythos, Cthulhu is a Great Old One, not an Outer God. Generally speaking, the difference is that a Great Old One is a planentary entity, while an Outer God is a galatic (or larger) entity.
- ↑ 1.0 1.1
 Fate/Zero - Volume 4: Prelati's Spellbook: Text of the Sunken Spiraled City
Prelati's Spellbook: Text of the Sunken Spiraled City
- ↑ 2.0 2.1 Fate/Grand Order Epic of Remnant - Salem: The Taboo Epiphany Garden: Epilogue ~Salem~
 Fate/strange Fake - Volume 4 - Servant status: True Caster [T]
Master: Francesca Prelati
True name: Francois Prelati
Gender: Unknown (summoned Saint Graph is male)
Height/Weight: 152cm 38kg
Alignment: Chaotic Evil
Noble Phantasm: A
■ Territory Creation: B
■ Tool Creation: B
■ Illusion: A
A skill that denotes excellence in the art of illusion within the field of magecraft. At this level, it is even possible to go beyond people and deceive the environment.
■ Disciple of Spirits: B
Proof that one has been initiated in magecraft by certain lake spirits. The efficacy of magecraft improves dramatically.
■ Divinity: E-
Prelati shares the blood of a deity, but as that deity is banished his rank is low.
Barely manifested at rank E due to combination with traditions that link Prelati with Beelzebub.
■ Grand Illusion: The Sunken Spiral Castle Does Not Exist, Therefore There is No End to This World's Madness
Type: Anti-Personnel Noble Phantasm
Maximum Targets: 10 people
Legends that Prelati revealed the form of Beelzebub to sworn friends or that he (she) was the incarnation of Beelzebub combined with the art of illusion and lineage that Prelati originally possessed and sublimated into a Noble Phantasm. It is a great magecraft that surpasses even environment to deceive the very texture of the world. It is even capable of causing a target to hallucinate that Prelati has imprisoned them in a Reality Marble. It is still an illusion, however, and therefore lacks the power of a Reality Marble.
■ Prelati's Spellbook: Textbook of the Sunken Spiral Castle (unusable)
Type: Anti- Noble Phantasm
Maximum Supplement: 1000 people
The result of Prelati utilizing magecraft while dispelling their own rationality with drugs Prelati compounded themselves. Against astronomical odds, it ended up linked to a place one must not link to, Prelati inscribed the truth of it in a blank bible that was a Mystic Code and sealed the link itself. Prelati was therefore incapable of reproducing it, and entrusted the spellbook capable of opening the one and only link to a knight who was their sworn friend. Prelati can never use the Noble Phantasm unless the knight returns it on the spiritual level, but will the day of their reunion ever come?
■ 道具作成： Ｂ
■ 精霊の弟子： Ｂ
■ 神性： Ｅ－
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Fate/Grand Order - Hokusai Trial Quest - Act 04: Edo's First Dream Chaldea's Real Dream
- ↑ Fate/Grand Order Epic of Remnant - Salem: The Taboo Epiphany Garden - Act 08:
- ↑ 6.0 6.1 6.2 Fate/Grand Order - Hokusai Trial Quest - Act 03: A Brushstroke of Spring Dream
- ↑ 7.0 7.1 Fate/Grand Order - Hokusai Trial Quest - Act 01: A New Year's Day Dream
- ↑ 8.0 8.1 Fate/Grand Order - Katsushika Hokusai (Foreigner): Dialogues
- ↑ Fate/Zero - Episode 02: "False Start"
- ↑ Fate/Zero - Episode 13: "Forbidden Banquet"
- ↑ Fate/Zero - Episode 14: "The Bloody Battle of Mion River"
- ↑ Fate/Zero - Episode 15: "Golden Radiance"