The Byakhee, or the Biyarky, are eldritch servants of the Great Old One Hastur the unspeakable. They are a race of three meter high hybrid creatures made of ordinary matter that live in interstellar space. A hybrid who is neither completely crows, nor moles, nor buzzards, nor ants, nor vampire bats, nor decomposed human beings, they can neither be fully perceived nor remembered by a sane person.
They can travel through space and carry a rider, but the rider must have something capable of protecting them from the cold and vacuum of space. They can be summoned to Earth to carry out tasks or act as steeds. Byakhee need not breathe, and do not appear on Earth unless summoned by sorcerers or called by their master, Hastur. Still, there is a chance that they will go down to any planet if they need to hunt for food.
"From the Cthulhu Mythos, they are lower servants of the king of the gods, Hastur. They can be summoned by blowing on a magic stone and chanting a special spell, and if one succeeds in summoning a Byakhee, they can instantly move whenever they wish. One can also ride a Byakhee into outer space to where Hastur is, but in that case the rider will need to drink a special golden mead that will protect them from the vacuum."
Biyarky is a rare demon that can be found in the fourth area of the Bomb Shelter. The only way to encounter it is to have Hastur equipped on a party member. When first encountered, a special conversation can happen between them and Hastur, gifting the Golden Honey item to the party. The Golden Honey prevents all enemy encounters for a while, but unlike the Estoma spell and the Clean Salt item, the Golden Honey will prevent encounters even against enemies of an equal or higher level than the party.
"A malformed, winged servant of the Cthulhu Mythos' cosmic god Hastur. They can fly even in the vacuum of space and it's said that if you can summon one, it will carry you wherever you wish."
—Persona 5 Royal Background
Byakhee is the twelfth Persona of the Moon Arcana, and can either be found within Maruki's Palace (in areas past the Auditorium) or otherwise available to be fused after 1/12. It never appears within Mementos.
When itemized by Electric Chair execution within the Velvet Room, Byakhee offers the Null Wind Skill Card, but with the Fusion alarm sounding, Byakhee offers instead the Reflect Wind Skill Card.
A Byakhee confronted Kasumi Yoshizawa during her, Joker, and Morgana's first accidental visit to the Palace in October, awakening Sumire's Persona Cendrillon. It was defeated by the trio. Unlike future Byakhees, this one has a weakness to Bless attacks, whereas the ones fought later are neutral to Bless attacks.
As a Shadow, Byakhee is referred to as "Evil Synthetic Organism." Takuto Maruki also summons these creatures to assist Cendrillon in battle when she attacks the Phantom Thieves.