The rule of 10 is a rule created by Aheyar, the god of logic. It's purpose is to prevent more than 10 immortals from existing on a single planet. Immortality in this sense means eternal life and immunity of all diseases that have negative effects. It does not in anyway mean you cannot be killed by ordinary means such as physical harm or poison. Immortality can be gained by the use of powerful spells, potions, artifacts, or being granted it by the gods.
Decacellpotris is a spell that has two functions. One is to remove someone's immortality, and the other is to trade someone's immortality for something of equivalent exchange such as someone's life. Decacellpotris is one of the rarest spells of all time, and it'd be surprising if a commoner even knew of it's existence, or anyone alive knowing how to cast the spell.