Morag Tong is a medium armor set. When you deal direct damage, you cause the enemy to take 10% more damage from all Poison Damage abilities for 5 seconds.
This set is often used in groups where several stamina setups are present. It increases all kinds of Poison Damage, such as from Abilities, Sets, Enchantments, etc. Morag Tong is also often only used on the front bar of a setup. The debuff on the enemy is applied for 5 seconds. Most rotations no longer have you playing on the backbar for more than 5 seconds, therefore you can almost guarantee an uptime of 90% on the enemy even when you only frontbar the set.
The Morag Tong set is part of the base game and can be acquired in Cyrodiil at the Bruma Elite Gear Vendor. Each set item from the Morag Tong set is bound on equip and you can acquire medium armor, jewelry and weapons. It comes in the three alliance motif styles (Ebonheart Pact, Daggerfall Covenant and Aldmeri Dominion).