The costume packs come in four separate sets, each costing $1 on Nintendo's eShop. They were initially made available to players as exclusive content, to those that had pre-ordered the game.
The first three packs contains costumes for Link and Zelda, and they take inspiration from other Zelda games - The Legend of Zelda: Twilight Princess, The Legend of Zelda: Ocarina of Time, and The Legend of Zelda: Skyward Sword.
The fourth pack is "The Demon King Costume Set" and includes costumes for Ganondorf from Ocarina of Time and Twlight Princess.
If you're yet to play Hyrule Warriors, and it looks like your kind of game, check out our review to find out more.