From publisher blurb:
Into The Darkness...
Scattered across ancient tomes and the whispers of reclusive mages lie the secrets of dark powers, designs of the immortal horrors humankind knows only as Fiends. These incarnations of cruelty and ruin offer wondrous secrets and incredible power, but never without a price.
The Diabolist's Handbook offers a myriad of potent abilities focusing on Fiendish themes of control, corruption, and ruin. Channel inhuman powers as a Champion Medium, Binder Troubadour, or Malefactor Symbiat. Draw upon Fiendish magics such as Distortion Aura, Necromantic Conscription, Entrapping Circle, Eternal Shackles, and Power of the Name. Explore new Damnation feats, crafting traditions, racial options for Tieflings and Aasimar, and variant rules that expand the uses of these systems. All these powers and more lie within the pages of the Diabolist's Handbook, open to all those who seek them.
For Pathfinder 1st edition and the Spheres of Power/Spheres of Might systems.