Some weapons are so intricately symbiotic with the wielder that death for the wielder typically means a similar case for the weapon. They won't exactly die (can't remember if I said otherwise) but they will fall into a state of inertia for an unspecified (but typically abnormally long) period of time when the right successor comes around and awakens them again. These sorts of weapons are rare and sentient to a high degree but the tradeoff is the frankly immense power they dish out.
Hezekiah's weapon is one such case, having existed long before Mikhail's time but inert up until Hez's prodigious ass came around.