Roar of Alkosh is a stamina based medium armor set. When you activate a synergy, you send a shockwave from your position that deals 1720 Physical Damage and an additional 12040 Physical Damage over 10 seconds. Reduces the Physical and Spell Resistance of any enemy hit by 3010 for 10 seconds.
Though a medium armor set, this set is mostly worn by tanks in PVE. The reason for this is that the 5-piece bonus offers a unique penetration debuff. Penetration plays a very important aspect in PVE. Because of Alkosh's unique penetration debuff, it stacks with all kinds of other penetration debuffs. This makes Alkosh a very valuable set as it allows well-organized trial groups to easily achieve the Penetration cap.
The reason mostly tanks run it is because of the uptime. It lasts for 10 seconds, and activates in front of a player whenever they use a synergy. Tanks are usually the ones standing in front of the boss, so their chance to hit is basically always 100%. Activating synergies is also an important part of resource management for tanks. Which means that with this set, tanks are automatically debuffing the boss while maintaining a healthy resource pool.
Of course this does not mean that Alkosh cannot be worn by damage dealers in PVE. It is still a very solid choice as a damage dealing set and can be worn by any stamina damage dealer in case the tank is not comfortable wearing it.
Roar of Alkosh is part of the Thieves Guild DLC. It can be found in the Maw of Lorkhaj Trial. You can acquire medium armor, weapons and jewelry of this set. It uses the Dro-m'Athra motif style. Each set item is bound on pickup.