Cleon
Legend
Umm, yes. That's a real McGuffin adventure!
So how tough do you want to make it to destroy the charms? Not worse than a phylactery, I'd think.
Well I don't think a standard charm needs to be as tough as a phylactery, but your proposed stats seem a bit flimsy - a Str 10 creature could snap one with their Break DC 20.
Maybe keep the 20 hit points and give it hardness 12, Break DC 24?