The Four Legendary Weapons(伝説武器) are later referred to as the Four Heavenly Weapons(聖武器) since many religions formed centering around the Legendary Heroes. The major religion in Melromarc is the Four Saints Faith that worship all the Legendary Heroes equally, but there is also Three Heroes Church, a radical faction those who dislike Demi-Humans, that only worships the Legendary Sword, Spear and Bow, excluding the Shield as its guardian of Demi-Human. The major religion in Silt Welt is the Shield Faith that worships the Shield Hero only. After the final wave the legendary weapons, except for Naofumi's Shield, gained the ability to be summoned and dispelled by their owners at any time. As a result of the world fusing with Glass's World, there became a total of 8 Holy Weapons since the world being a fusion of 8 different original worlds. While the new Heroes of the Legendary Weapons are summoned in later generations, the Shield has not reappeared due to Naofumi still possessing the Shield as a God.

The Legendary Weapons have 3 basic ability categories that appear by default which indicate the weapon has been strengthened:

Category Description
Weapon transformation The legendary weapon takes on a different shapes and abilities. In order to perform weapon transformation, the owner must place their hand on the weapon and call for the desired weapon by name.
Equipment bonus Equipment bonuses are stat bonuses, abilities, and or skills that are gained after certain weapons are unlocked. When the weapon is unlocked, the user gains these bonuses forever. (Passive Abilities)
Special Effect An exclusive effect that will only be usable when equipped with a specific weapon. The effect won't be usable when weapons other than the specific weapon is equipped.(Active Abilities)

Other strengthening methods can be unlocked by simply believing they work.

Forms of Strengthening
Energy for Reinforcement is indicated as proficiency and displayed as fraction which indicates the ability of the user with that weapon. If a weapon is used repeatedly the user gains proficiency and can later choose to turn proficiency into skill points using "Energy Grant" when the proficiency reaches the max.
Energy Grant is hidden power that can be unlocked by using energy granted from resetting the weapon's skill proficiency. Additionally, items absorbed into the weapon can be converted into energy.
Rarity By using energy, it is possible to increase the weapon's rarity. With an increased rarity the weapons overall ability will improve. Rarity levels include Common (C), Uncommon (UC), Rare (R), Super Rare (SR), Super Rare+ (SR+), and Artifact (AF).
Skill Points are points used to change the rarity of the item or to awaken the real power of weapon. Using the "Energy grant" some items can be turned into energy and skill points.
Tempering is the strengthening of equipment through reforging it using ore.
Spirit Enchantment is simply giving a weapon the fragment of a demon's soul to absorb (absorbing defeated monsters) through which special powers can be granted.
Status Enchant Stats can be increased.
Reinforcing is increasing the weapon's power by applying a specific type of ore.
Body Modification Using an energy stored up besides EXP, the user can additional points to your Stats. Like in some RPGs, how certain seeds and other items can permanently raise a stat. It seems plain at first, but the amount of points you can distribute is high. Its main advantage is that the user can get an additional bonus on top of those for the weapon you have equipped.
Weapon Synthesis Allows the user to transfer special abilities from one weapon to another. new weapons can be unlocked using it. The amount of syntheses that can be performed depends on the weapon.
Skill Enhancement User can distribute skill points into Skills to increase their output.
Magic Enhancement User can distribute skill points into Magic to increase their output.
Magic and Skill Proficiency Level By using the same skill again and again, a point value are stored up, and the output and ease of use increases.
Growth Elevation While it may increase the innate amount of stats gained during a level without affecting weapons, it decreases the level of the person it's used on in proportion. That person will have to level up all over again.
Strengthening by Money By expending your wealth, you increase the bonuses brought by other weapons (the other forms of strengthening).
Item Encyclopedia Bonus Increases stats based on how much of a certain type of item has been placed into the weapon.