The Longbow of Diana should create arrows on-the-fly as a passive effect when it is fired, rather than creating a stock of +0 arrows as an invoke ability. The arrows could be ordinary arrows with an enchantment equal to the Longbow’s, but it’s probably more interesting to clone arrows from your quiver (assuming the thing you have quivered is suitable to be fired from a bow), as this lets you keep multishot bonuses from using same-race ammo or use silver arrows and preserves the incentive to have well-enchanted ammo. Cloned arrows should always be destroyed when they hit the ground, so you can’t use it to create hundreds of highly-enchanted arrows.

A slightly different way to take this idea would be that arrows don’t get created on the fly, but automatically return to your quiver when they hit the floor. This means that any arrows that get eroded by hitting a rust monster or something will return to you eroded, and any arrows that get mulched will still be lost, preserving the ammo management aspect of the ranged combat game. On the other hand, it could be argued that removing ammo management for the player is a great benefit for a quest artifact to have.