Swarm Mother is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Swarm Mother (unique) style.
The 1-piece set bonus adds 129 Stamina Recovery.
2-piece set bonus: When you hold block an attack from an enemy that is further than 5 meters away from you, you spin strands of spider silk to pull the enemy to you. This effect can occur once every 1 second.
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 I 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.
This is a very popular tanking set for players who want the set bonus in addition to slotting a pulling ability because it costs no resources, there is no cooldown, and it procs upon blocking which tanks do during trash pulls anyway.
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.