The Lamplighters League

Tempered Weapons are an unlock from character's skill tree and not an equipment. They are found deep within the skill tree and require 12 skill points to unlock as well as the skills in the path leading up to the Tempered Weapon skill. In addition, they require you expend one Seric Steel. This resource can be gained as a mission reward for certain critical missions such as raids, or on high difficulty missions later in the game.

The achievement unlocks when you unlock the skill.