Ataraxia is a magic sword created by Nesta Archeron.
The sword is simple and made of steel that seems to glow.
Cassian had taken Nesta to a blacksmith in Velaris so she could learn how weapons were made, the hard work that came with creating them, and thus appreciate them before actually wielding them. Nesta created the magic sword unknowingly while practicing hammering at the metal for a while infusing it with her power of silver flames while she made them.
Later on, the sword was used to slay Lanthys, a creature that was truly immortal.
The name Ataraxia means Inner Peace in the Old Language.