Here is my first revision. I changed the prerequisites and swapped the abilities at some levels.
I used the base save bonus instead of a base attack bonus, because with a base attack bonus fighters can enter this prc too easily.
Prerequisites
Alignment: Lawful evil or lawful neutral (but high-ranking members are always evil)
Skills: Diplomacy 8 ranks, Sense Motive 8 ranks, Bluff 6 ranks, Listen 8, Move Silently 8, Disguise 4, Knowledge (religion) 4 ranks, Knowledge (the planes) 4 ranks
Base Save Bonus: Will +4
Feats: Improved Unarmed Strike, Combat Reflexes, Negotiator, Sneak Attack +2d6
Spellcasting: Ability to cast divine spells of 1st level or higher
Special: Must have made a peaceful contact with a devil and must worship a devil as god.
Benefits
HD: d8
Skill points: 6 + Int modifier
Class Skills: Bluff (Cha), Concentration (Con), Craft (any) (Int), Decipher Script (Int), Diplomacy (Cha), Disguise (Cha), Escape Artist (Dex), Forgery (Int), Gather Information (Cha), Heal (Wis), Hide (Dex), Intimidate (Cha), Jump (Str), Knowledge (Arcana) (Int), Knowledge (Religion) (Int), Knowledge (Planes) (Int), Listen (Wis), Move Silently (Dex), Open Locks (Dex), Pick Pocket (Dex), Profession (any) (Wis), Sense Motive (Wis), Sleight of Hands (Dex), Search (Int), Spellcraft (Int), Spot (Wis), Tumble (Dex)
Base Attack: 3/4 (as cleric)
Good saves: Reflex and Will (as bard)
Level Special Abilities
1 flurry of blows, Devils claws, +1 spellcasting level
2 Ki strike (magic), +1 spellcasting level
3 Sneak attack +1d6
4 Poison immunity, +1 spellcasting level
5 blindsense 10 ft., spellcasting level
6 Sneak attack +2d6, +1 spellcasting level
7 Ki strike (lawful), +1 spellcasting level
8 Telepathy 100 ft, greater flurry
9 Sneak attack +3d6, +1 spellcasting level
10 Death attack, see in darkness +1 spellcasting level
Special Abilities
Flurry of blows: The devil's fist´s level stack with his monk levels (if any) for the purpose of determining his flurry of blows attack bonus. If a devil´s fist does not have a monk levels prior becoming a devil´s fists treat him as a monk of the same class level as his devil´s fist class level for determining the flurry of blows attack bonus.
Devil's Claw: The hands and feet of a devil's fist transform into hands and feet with retractable claws. These claws deal unarmed damage per the chart above. A devil's fists with natural claws that already deal as much or more damage gain only the ability to retract their claws.
The claws do unarmed damage 1d8 at level 1, 1d10 at level 4, 1d12 at level 7, 2d6 at level 10.
Sneak Attack: As the rogue ability.
Ki Strike: As the monk abilities.
Poison Immunity: Devil's fists are immune to all natural and manufactured poisons.
Blindsense: The devil's fist gain the ability to sense creatures within 10 feet without making a Listen or Spot check (though they still have total concealment).
Greater Flurry: As the monk ability
Telepathy: As the diabolic ability.
Death Attack: As the assassin ability.
See in Darkness: Like some devils, devil's fists can see perfectly in darkness of any kind, even that created by a deeper darkness spell.