Selene is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Selene (unique) style.
The 1-piece set bonus adds 1096 Maximum Stamina.
2-piece set bonus: When you deal direct melee damage, you have a 15% chance to call on a primal spirit that mauls the closest enemy in front of you after 1.3 seconds for 12000 Physical Damage. This effect can occur once every 4 seconds.
The Shoulder can be acquired by using an Undaunted key on Glirion the Redbeard's chest. The Shoulder type, weight and trait are RNG based and can be from any dungeon this NPC gives Pledges for.
The Helm can be acquired from Selene's Web which is part of the base game and located in Reaper's March. The Helm is guaranteed to drop from the final boss in veteran mode only. The weight and trait are RNG based.
You can mix-and-match Monster Set shoulders and helms from different sets and add two 1-piece bonuses to your build, or wear a complete set to take advantage of the full 2-piece set bonus.