While Hastur is best known as part of the Cthulhu Mythos, he was originally created by Ambrose Bierce in the short story "Haita The Shepherd." He later appeared in one work by HP Lovecraft, but is best known for his role in August Derleth's stories. Hastur is the half-brother of Cthulhu.
He is also known in other names:
The Feaster from Afar, a black, shriveled, flying monstrosity with tentacles tipped with razor-sharp talons that can pierce a victim's skull and siphon out the brain
The King in Yellow
He Who Is Not To Be Named as saying Hastur's name three times in a row (Hastur Hastur Hastur) brings the chanter death.
The King In Yellow is also the title of a fictional stage play within the Cthulhu Mythos, where Hastur is mentioned, as well as a real-life short story collection in which the stage play appears several times. Hastur's Material Card features the cover of this book.
"One of the gods who appear in the Cthulhu Mythos, known as "The Unspeakable." He is the chief of the gods of the winds and the universe, and lives on the star Celaeno, located in the Hyades star cluster. There are many accounts on Hastur's appearance, but the most common one is that he has a body covered with black scales, boneless arms, and a fish-like head. Hastur has many powers, including possessing human corpses and flying freely in outer space. He is Cthulhu's half-brother."
—Persona World
Hastur appears as a Persona in Persona 2: Eternal Punishment. This is a secret persona whose material card, The King in Yellow, cannot be acquired by normal means. The game itself does not provide information about its existence. To get it, the party must talk to the Wang Long-obsessed girl in 2X Slash, located in Aoba, and say "HASTURCOMEFORTH" (イアイアハスタア, Ia Ia Hasutā)? as their birth month when prompted. After that, Hastur's material card will be found in a package by Maya Amano's desk, in Kismet Publishing.
Persona 5 Royal
"An evil god of the Cthulhu Mythos and one of the eldritch deities known as Great Old Ones. It wields great power over wind. Also called "The Unspeakable" and "The King in Yellow.""
—Persona 5 Royal Background
Hastur is the eighth Persona of the Star Arcana, and can either be found during the Third Semester Arc within Maruki's Palace or otherwise available to be fused after 1/12. As a Shadow, it is known as "Warped Abyss". One appears as Takuto Maruki's right hand in the palace's first investigation, and attacks the protagonist and Goro Akechi to stop them from taking Sumire Yoshizawa back from Maruki. Proper encounters with this Shadow will occur after the Monitoring Room.
Hastur is the only Persona to learn Abyssal Eye, an Almighty Attack that does Severe damage to all foes. This skill is exclusive to it and cannot be transferred to other Personas. When Itemized by Electric Chair execution, Hastur offers the Spiral Gale Ring accessory, which allows whoever wears it the ability to cast Magarudyne.
These are all quotes used by the boss variant encountered in the 2nd of January. The regular enemy version encountered further into Maruki's Palace use the same dialogue as other male enemy Shadows.
"Stubborn imbeciles, rejecting our lord..." (Start of battle)
"Come!"
"Kneel..."
"How's this?"
"Fear me!" (Using Evil Touch)
"No mercy!" (Using Abyssal Eye)
"Soon..." (Party member at low HP)
"Fools!" (Hastur dodges)
"No... Such pathetic attempts won't be enough!" (After reduced to low HP)
"Accept his mercy... Give yourselves over to him!" (Before battle ends)
Tower
