Infernal Guardian is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Infernal Guardian (unique) style.
The 1-piece set bonus adds 1096 Maximum Magicka.
2-piece set bonus: When you use a damage shield ability, you have a 50% chance to lob 3 mortars over 2 seconds at the furthest enemy away from you that each deal 5500 Flame Damage to all enemies within 5 meters of the blast area. This effect can occur every 6 seconds.
The Shoulder can be acquired by using an Undaunted key on Glirion the Redbeard'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 City of Ash I which is part of the base game and located in Greenshade. 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.