Scourge Harvester is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Scourge Harvester (unique) style.
The 1-piece set bonus adds 1206 Maximum Health.
2-piece set bonus: When you take damage, you have a 6% chance to create a beam that steals 5805 Health over 4 seconds from the attacker. While the beam holds, you gain Major Vitality, increasing your Healing Received by 30%. The beam breaks if the enemy moves further than 8 meters away. This effect can occur once every 6 seconds.
The Shoulder can be acquired by using an Undaunted key on Maj al-Ragath'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 Wayrest Sewers II which is part of the base game and located in Stormhaven. 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.