Mongol Helmet is a helmet you receive while playing "Fit for the Khan", from Tales of Tsushima. It is a story reward, so the tale is the only way to get it.
Play "Fit for the Khan". The helmet is handed over during the tale, not at a shop.
Conqueror's Mask
Conqueror's Mask is a mask you get by finishing "Fit for the Khan", from Tales of Tsushima. It is a story reward, so playing the tale is the only way to add it.
Image: Sucker Punch Productions / PlayStationMongol Commander's Armor
The Mongol Commander's Armor is a disguise set you earn from Fit for the Khan. Out of combat it makes Mongols much slower to spot you, and it adds health and damage reduction, which makes it the toughest set in the game.