From Dark and Darker Wiki
Weapons can improve and become stronger as their rarity level increases. A weapon's rarity typically reflects its level of power, with higher rarity weapons providing more damage output and other attributes that can enhance the player's performance in combat.
There are 7 rarities in the game.
Poor | Common | Uncommon | Rare | Epic | Legendary | Unique
Uncommon rarity adds up to 1 new enchantment.
Rare rarity adds up to 2 new enchantments.
Epic rarity adds up to 3 new enchantments.
Legendary rarity adds up to 4 new enchantments.
Unique rarity adds 5 enchantments and may add a special effect varying based on weapon. A list of these effects can be found at Artifacts