The Ancient Armour can be obtained by completing The Isle of Apples quest and sailing to Avalon. Once you arrive on the island, it can be removed from an armour stand inside the solely shop. The armour can be taken from the stand without any consequences. The armour is needed for the Last Legionairre achievement
It is comparible to chain mail made with the Crafting skill using hardened as additional component to make the armour immune to burning. The armour also has a 10% chance to revive the player character when he is incapacitated, which is an unique property.