The Diabolic domain was a suite of spells granted by entities, not necessarily of divine nature, who were of infernal nature. It granted divine spellcasters like clerics with spells and other powers related to devils, the Hells, and the planes.
A priest with the Diabolic domain was capable of supernaturally becoming more persuasive, deceptive, intimidating, or better at seeing through people. However, they could only empower one of these interpersonal skills once a day.
As they advanced, a priest with this domain could acquire the traits and prowess of a devil, such as gaining a battle-ready tail with devil's tail (the later version replaced this with a simple protection from good), seeing through even magical darkness with devil's eye, and gaining a devil's charisma and otherworldly nature with devil's ego. They could also command hellfire, creating explosions of brimstone with hellfire and hellfire storm. Finally, they could issue a demand over a distance and summon and bind outsiders with lesser and regular planar binding. At the pinnacle, they could travel between planes directly with gate or gain the powers of a pit fiend with investiture of the pit fiend.