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.
Hastur appears as a Persona in Persona 2: Eternal Punishment. He may not differ from the others gameplay-wise, however, he stands out since his material card, The King in Yellow, cannot be acquired by normal means. 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.
As of the present day, there is still no way to know which demon tells the party about it or how the party was supposed to know how to summon Hastur outside of this method.
"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". Takuto Maruki also summons one to attack the protagonist and Goro Akechi in the palace's first exploration session for rejecting his deal. 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.