Storm Master is a medium armor set. When you deal Critical Damage with a fully-charged Heavy Attack, your Light Attacks deal an additional 1333 Shock Damage for 20 seconds.
The Storm Master set is part of the base game. It drops in Tempest Island in Malabal Tor. Each set item is bound on pickup. You can acquire medium armor, weapons and jewelry of this set in the Ancient Elf motif style.