Flanking Strategist is a medium armor set. The 5-pice bonus adds 400 Weapon Damage to your damaging abilities when you use them to attack an enemy from behind or their sides.
Flanking Strategist is part of the Dark Brotherhood DLC and drops in the Gold Coast in the Overland content. You can also find it on Guild Traders. Each set item is bound on equip. You can acquire medium armor, jewelry and weapons for this set in the Order of the Hour motif style.