Earned by fusing a Chaos Crystal with at least 50 charges and a melee weapon.
The sword will be unidentified and always require 14 strength to successfully wield. Upon attacking enemies a few times, the sword will be identified as enchanted by a random enchantment. This sword can then be repeatedly upgraded, boosting the enchantment every tier (+1,+2 etc.) as well as further reducing the requirement to wield, increasing the efficiency.
It is recommended that if there is a specific enchantment desired, save the game and then monster hunt until the desired enchantment is reached. Also note that infusing any cursed melee weapon will remove the curse and generate an entirely new weapon whilst consuming the old one, so no need to worry about cursed chaos weapons.
Is it normal for the chaos sword to change enchantment when it is upgraded?
I have a vampiric chaos sword +2, and when it upgrades, sometimes it becomes something else. Bug or feature?
It is a feature, it is Chaos weapon after all