Soawn of Mephala is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Spawn of Mephala (unique) style.
The 1-piece set bonus adds 1096 Maximum Stamina.
2-piece set bonus: When you deal damage with a fully-charged Heavy Attack, you create a web for 10 seconds that deals 1100 Poison Damage every 1 second to all enemies within 4 meters and reduces their Movement Speed by 50%. This effect can occur once every 10 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 Fungal Grotto II which is part of the base game and located in Stonefalls. 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.