Maw of the Infernal is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Maw of the Infernal (unique) style.
The 1-piece set bonus adds 1206 Maximum Health.
2-piece set bonus: When you deal damage with a Light or Heavy Attack, you have a 10% chance to summon a fire-breathing Daedroth for 15 seconds. The Daedroth's basic attacks deal 4257 Flame Damage. This effect can occur once every 15 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 Banished Cells II which is part of the base game and located in Auridon. 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.