Almost all true devils have the power to make and sign contracts with mortals.  The boons granted by the contract may come from the devil's personal power, or in the case of very low level devils, it may be sponsored by one of its superiors.  Either way, the wording of the contract and source of power and responsibilities may be very convoluted, but are always weighted to benefit the devil.

Hierarchy of Devils

Devils of the Nine Hells

Name Power Domain Portfolio Allies Devotees
Amon Duke (25) Avernus (1) War, Violence, "The Destroyer    
Thamuz Duke (25) Dis (2) Emotions, Desires, Greed    
     Furcas (20) Dis Rhetoric, Knowledge    
     Dispater (20) Dis (City)      
Moloche Duke (25) Minaros (3) Knowledge, Secrets, Curses   Witches
Alastor Duke (26) Phelegethos (4) Cruelty, Pain, Death, "The Executioner"    
Baal Duke (26) Stygia (5) Storms, Elements   Witches
Shax (f) Duke (27) Malboge (6) Illusions, Dreams, Magic   Witches
Astorath Duke (27) Maladomini (7) Hopelessness, “The Imprisoner”    
Malcanthet (f) Duke (28) Caina (8) ...    
Druaga Arch, Lord (30) Nessus (9) Power, Deceit, "Lord of Lies"