Lord Warden is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Lord Warden (unique) style.
The 1-piece set bonus adds 2975 Physical Resistance and 2975 Spell Resistance.
2-piece set bonus: When you take damage, you have a 50% chance to summon a shadow orb for 10 seconds that increases the Physical and Spell Resistance of you and your allies within 8 meters by 3870. This effect can occur every 10 seconds.
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 Imperial City Prison 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.