A diamond creature is made of living diamond, making it an incredibly tough foe able to shrug off attacks and soak a tremendous amount of damage. Most diamond creatures are denizens of the deepest sections of the elemental plane of earth, or special mystic guardians created by the gods to protect special sites and holy places.
The diamond creature’s body is hard as diamond. This gives the diamond creature a bonus to natural armor equal to its Constitution or Charisma bonus (whichever is higher), and 10 temporary hit points per Hit Die. The temporary hit points are restored at dawn each day, but cannot otherwise be healed.
The creature can grow a long, sharp blade from its diamond body. This acts as a keen scimitar of speed with which the creature is proficient, of the same size as the base creature, which cannot be disarmed or sundered. The diamond blade gains a +1 enhancement bonus for every 5 Hit Dice of the base creature, to a maximum of a +5 bonus. If the base creature’s Dexterity bonus is greater than its Strength bonus, it also gains Weapon Finesse as a bonus feat (if it doesn’t already have it).
Notes: When calculating the diamond creature’s diamond blade attack, take its base attack bonus, add its Strength or Dexterity bonus (whichever is higher), and the weapon’s enhancement bonus (if any). When making a full attack routine, remember to allow the diamond creature two attacks with the diamond blade at its full attack bonus, due to the weapon’s speed special ability. If this is strictly superior to the base creature’s existing attacks, simply remove them.