Invoking the Longbow of Diana gives you light arrows. These are a special object, weightless, created with the same enchantment as the Longbow, but attempting to enchant them further will fail (maybe making the enchanted arrows evaporate). If they leave your inventory for any reason, they will vanish. As a weapon, they are very powerful (to the point of probably instakilling most weaker monsters they hit). To prevent the player stockpiling them, the invoke effect will only top up the amount of them in your inventory to 5.

When a light arrow strikes a target or falls to the ground (and vanishes in the process), it permanently lights up the surrounding area.