Master Architect is a magicka-based light armor set. When you use an Ultimate ability, you and the closest 2 allies within 28 meters of you gain Major Slayer for 10 seconds, increasing your damage done to Dungeon and Trial Monsters by 15%.
This set is especially popular with classes which have low-cost Ultimates such as Incapacitating Strike/Soul Harvest from Nightblades, Crescent Sweep/Empowering Sweep from Templars or the Bear ultimate from Wardens. Those ultimates only cost 70 ultimate points and you can almost get a 50% uptime on Major Slayer on your character with those Ultimates.
The Major Slayer buff goes to the 2 allies closest to you, so you have to make sure to coordinate with your group members, otherwise it might be that the tank or healer ends up with the Major Slayer buff instead of a damage dealer.
The Master Architect set is part of the Morrowind chapter and can be found in Halls of Fabrication in Vvardenfell. Each set item is bound on pickup. You can acquire light armor, weapons and jewelry of this set in the Clockwork motif style.