Feedback on magic system

I'm in the process of worldbuilding in my spare time, and came up with (I hope) a somewhat novel magic system.

Magic is born of the life force of a person. They can essentially borrow this power momentarily. If a person remotely throws a rock, say ten paces away from them, in an instant they expend the power they would've by walking over, picking the rock up and throwing it by hand. Key constraints are that they cannot create matter or energy out of thin air.

In my world, stuck in a somewhat medieval setting, this magic isn't known, but it exists. I feel like this opens up a bunch of possibilities. A desperate knight may unconcsiously reach into this magic to deliver a final, killing blow at the cost of their own life, and "heroic figures" in history may have achieved their feats by dipping into this magic and shortening their own lifespans as a result, whether knowingly or unknowingly. Slaves could be bought and sold for the express purpose of expending their magic, and general health and fitness could essentially strengthen your magic.

My world has experienced an ancient, cataclysmic event. In my head, this "magic" was a known quantity to civilizations before this event, and perhaps the event was caused by abuse of this magic. Maybe the power of thousands of slaves was expended at once and whoever was in charge of controlling this magic, couldn't do so. In any event, this shattering wiped the slate clean, and use of magic has been unconscious at best ever since.

This also ties to gods and deities. Although gods in the usual sense don't exist, widespread and concerted worship can give rise to such a power. In a similar vein, some gods could be fed by human sacrifice.

How's this sound to you? Any pitfalls I should be aware of, or ideas for how such magic could be used?