Fierce Deity is a form Link takes upon in The Legend of Zelda: Majora's Maskafter putting on the Fierce Deity Mask. Its power is derived from a great, powerful being that passed into a mask. This form is commonly referred to by fans as Oni Link.
The form enables Link to achieve his adult size and gives him much greater power and range. The two-handed helix sword limits his ability to wield a shield however. He can also fire beams of energy from his sword.
The Fierce Deity Mask can only be obtained if the player obtains all 20 non-transformative Masks and trades them to the 4 moon-children. A final child will give Link the mask after he notices all the other masks are gone. This form can only be accessed in boss battles however.
Hyrule Warriors - It is incorporated into Young Link's moveset as his Focus Spirit. This form gains the ability to fire Sword Beams but is mostly the same moveset-wise as regular Young Link. His Focus Attack features him slicing the Moon in half, damaging enemies around him. It is also a costume for Link in the Majora's Mask DLC
Majora's Mask 3D gives two new locations to transform in, being the Fishing Holes. However, when trying to leave, the owner will stop the player, wondering how Fierce Deity Link got in.
In Majora's Mask, the Fierce Deity is 2.2-meters tall, but Hyrule Warriors, this form is only 1.4-meters tall