Akaviri Dragonguard is a heavy armor set. The 5-pice bonus reduces the cost of your Ultimate abilities by 15%.
15% is actually a huge chunk of your ultimate, especially with high-cost ultimate abilities like Standard of Might, Elemental Storm, Barrier, or the Warhorn ultimate. These all cost 250 ultimate points, but Akaviri Dragonguard will give you a 37.5 point deduction, bringing their cost down to 212.5. This set is most commonly used on tanks to get their Ultimate ability ready a lot faster.
Akaviri Dragonguard is part of the base game and drops in Eastmarch in the Overland content. You can also find it on Guild Traders. Each set item is bound on equip. You can acquire heavy armor, jewelry and weapons for this set in the Nord motif style.