[HW] How do I acquire the Magical Sword through Adventure Mode?

I was scrolling through the Zelda Wiki and saw this banger image of Fierce Deity Link with the Magical Sword, so I naturally looked up "Magical Sword Hyrule Warriors". The only info I got was that it's unlockable through Adventure Mode, but it didn't say where in AM. Can someone tell me where I might be able to find it? Thanks.