You can equip Din's Fire to a C button, but it uses up your magic meter, so you'll need a green potion to replenish it. Din's Fire is a popular strategy when defeating Dark Link, as he can't block it.
When using Din's Fire, Link puts a hand on the ground and is protected by a huge sphere of fire. Aside from the Dark Link battle, you can use this to light torches and burn enemies if they surround you.
Din's Fire has also appeared in Super Smash Bros. Brawl and Super Smash Bros. Melee as one of Zelda's attacks, to name its non-canon appearances.
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