The Valkyrie Helm is a hat that can be found on the first floor of the Clothing Store. It can be purchased for 42 Bucks. After purchasing it, you'll only be able to wear it in Adopt Me!, unless you buy it on the Roblox Catalog.
The description of this item on the Roblox Catalog reads:

This splendid helm is a replica of ones worn by the valkyries, a group of minor deities who serve Odin in Norse mythology. Led by Freya, they uphold justice and righteousness.