Agility is a weapon, shield and jewelry based 3-piece set. You get 1451 stamina from the 2-piece bonus and 193 weapon damage from the 3-piece bonus.
Agility is most often used as a weapon piece only to gain the huge amount of extra stamina.
The Agility set is part of the Imperial City DLC. You can acquire it through turning in 60 Key Fragments in Imperial City at Monstrous Tooth Vault or Clawed Trophy Vault. You can also get the set via the Daily Dungeon Rewards from using the group finder tool. Only weapons and jewelry exist for this set. It uses the Xivkyn or one of the Alliance motif styles.
ESO
Sets