War Machine is a stamina based medium 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%.
War Machine is especially popular on 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 War Machine set is part of the Morrowind and can be found in Halls of Fabrication. You can acquire medium armor, weapons and jewelry of this set. War Machine uses the Clockwork motif style.