Molag Kena is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Molag Kena (unique) style.
The 1-piece set bonus adds 129 Spell Damage and 129 Weapon Damage.
2-piece set bonus: When you deal damage with a 2 consecutive Light Attacks, you trigger Overkill for 6 seconds which increases your Weapon and Spell Damage by 516 but also increases the cost of your abilities by 20%.
The Shoulder can be acquired by using an Undaunted key on Urgalarg Chief-Bane'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 White-Gold Tower which is part of the Imperial City DLC and located in the Imperial City. 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.