The Iron Hoof is a gift of hell that is bestowed upon a character as part of a deal which involves a soul or a substantial service/sacrifice to the infernal powers. By turning the left foot of a character into a massive hoof of iron, the Lord of the Pit transfers some of its power to rule over the world upon said character.
LotFP compatible rules: when the character shouts a one-word order to a non-lawful character while stomping down with his or her Iron Hoof, the order is treated as a cleric´s Command spell, with a level equal to that of the character. Lawful characters are unaffected by the command, and the Saving Throw that a target may be entitled to (see spell description) is one against Devices (instead of Magic).
The Iron Hoof may be used once per day. Further uses (successful or not) call for Save vs. Magic on behalf of the character. On a failure, a gate to hell opens up behind the character, and he or she is dragged into it within a mere moment by monstrous arms while the choirs of the damned wail and howl (the character is GONE).