Engine Guardian is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Engine Guardian (unique) style.
The 1-piece set bonus adds 129 Health Recovery.
2-piece set bonus: When you use an ability, you have a 10% chance to summon a Dwemer Automaton to restore 1162 Health, Stamina or Magicka every 0.5 seconds for 6.5 seconds.
This is a popular set for tanks since they draw from all 3 resource pools. It is more popular in PvP than in PvE. The proc is RNG based so you cannot predict which Attribute it is going to restore when it procs. Because of this, many players prefer Monster Sets with more reliable/predictable procs.
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 Darkshade Caverns II which is part of the base game and located in Deshaan. 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.