Blood Spawn is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Blood Spawn (unique) style.
The 1-piece set bonus adds 129 Stamina Recovery.
2-piece set bonus: When you take damage, you have a 6% chance to generate 14 Ultimate and increase your Physical and Spell Resistance by 6450 for 6 seconds. This effect can occur 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 Spindleclutch II which is part of the base game and located in Glenumbra. 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.