Archer's Mind is a stamina-based medium armor set. The 5-piece bonus increases your critical damage done by 5%. Increases your critical damage done by an additional 10% when attacking from Sneak or Invisibility.
This is a very niche set and is sometimes used by gank builds in Cyrodiil due to the increased critical damage done.
The Archer's Mind set is part of the base game and can be found in the Dragonstar Arena in Upper Craglorn. Each set item is bound on equip. You can acquire medium armor, weapons and jewelry of this set in the Wood Elf motif style.