So I just dropped 20000 gold on this Void Crystal item, which claims to drain enemies of their magical energy and also grants me access to a powerful charging attack. However, it's unclear to me what sort of enemies in the game are considered to have magical energy, and what happens to them as a result of the Void Crystal's power. If I attack an enemy who has a projectile attack, will they be temporarily unable to attack me? What sort of situations is the Void Crystal useful in?
The Void Crystal is an interesting weapon that doesn't look like it has a lot going for it. It doesn't improve your normal attack at all, and its charged attack is strikingly similar to that of the Fire Crystal, but with no elemental affinity and it does normal damage. While on the one hand, this gives it the benefit of at least neutral damage, you should have access to enough elemental crystals to always be picking the most powerful one.
Why use the Void Crystal? Because its charged attack negates magical projectiles. Magical projectiles are specifically the ones that target your magical defense (so in general, these hurt a lot more). The common ones you run into are:
It does not work against
Everything else is pretty much fair game. You can tell if you cancel an attack because it'll make a glowy particle effect as it dispels it.
It has a short duration and it also has an awkward area, so it is a far better idea to perform this at an angle, rather than directly into the projectile. It is excellent if you don't need to focus on strict attack power (since it gives none), as well as if you need to approach particularly dense groups of magical opponents.
I'll include notes about specific bosses below, but since that's very specific information, it will be included in a spoiler block. Don't read this unless you have an idea why I'd make specific points about projectile attacks.