Whirling Grindstone

A whirling grindstone made of flint appears in an unoccupied square within 30 feet. The grindstone shreds flesh and shoots sparks. Attempt an impulse attack roll against the AC of a creature adjacent to the grindstone. On a hit, the creature takes (((your level − 4) ÷ 5, rounded down) + 2)d6 slashing plus (((your level − 4) ÷ 5, rounded down) + 1)d6 fire damage (or double damage on a critical hit). The grindstone lasts until the end of your next turn, and you can Sustain the impulse up to 1 minute. On subsequent turns, the first time you Sustain the impulse that turn, you can roll the grindstone up to 20 feet and can repeat the attack.

A creature within reach of the grindstone can Interact with it to sharpen a metal weapon. This grants a +2 circumstance bonus to the next damage roll made with that weapon within 1 minute.

Level (+5) The grindstone's damage increases by 1d6 slashing and 1d6 fire, and the bonus to weapons increases by 1.