Yes, there are some rules for making weapons and equipment, to stop OP happening in role play.
1. If you are a Demi-God, you must wait until your respective parent grants you the weapon/item/equipment.
2. You must be original with your ideas. No copying other character's weapons. For example: Poseidon's trident. Only Poseidon can have this item.
3. Not all weapons are invincible. They all have flaws. Remember this when making them. Characters like Athena can make more durable weapons because she can craft items such as weaponry.
That is all. Please follow them, and have fun.
~Athena