"Equip a Legendary Armor SET
Quick and Easy way to get this is to buy a gear pack from the in-game store.
Probably your first/best FREE chance to get this is when you finally eliminate all 5 underling cultists in the Eyes of Kosmos branch of the Cult of Kosmos. You should get all 5 pieces of the Snake Set
of Legendary armor from those cultists. The Sage of each branch carries a legendary weapon, not armor; so the Sage is not necessary to get this achievement.
Head = Viper's Hood
Arms = Venom Gloves
Torso = Scaled Torso
Waist = Slithering Belt
Legs = Noxious Boots
All the Legendary Sets that I've seen:
So, wear all 5 pieces (of the same set!). Achievement!
Try to wield a Legendary melee
weapon at the same time to earn this also