Domihaus is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Domhaus (unique) style.
The 1-piece set bonus adds 1096 Max Stamina and 1096 Max Magicka.
2-piece set bonus: When you deal damage, you have a 15% chance to create either a ring of fire or a ring of molten earth around you for 10 seconds, which deals 1000 Flame Damage or 1000 Physical Damage every 1 second. Standing within the ring grants you 200 Spell Damage or 200 Weapon Damage. This effect can occur once every 15 seconds.
The Shoulder can be acquired by using an Undaunted key on Urgalag Chief-Bane'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 Falkreath Hold which is part of the Horns of the Reach DLC and located in Craglorn. 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.