A Foil item (formerly Relic) is an item that can be found by opening a Relic Vault within a Reliquary map area (not to be confused with the Relic Chamber Map). These can be accessed with a Reliquary Key, rare items that have been temporarily introduced throughout the game.
Search from Path of Exile Trade.
Voidborn Reliquary Key Foil items are available with the following colour gradients.