Arena Ferox is the Fire Emblem: Awakening stage on Super Smash Bros for 3DS. It's essentially a Fire Emblem equivalent of Pokemon Stadium and Pokemon Stadium 2. You see, the stage is a basic platorm area that changes environments throughout the battle. But since Fire Emblem doesn't really have any types like Pokemon, they're all just assortments of platforms and objects. Well, Fire Emblem DOES have types, but I don't know how they'd make an axe-type area or a bow-type area. Also, one of the layouts brings back the statues from Castle Siege, so that's pretty cool.
Arena Ferox is also the only Fire Emblem stage in Super Smash Bros 4 (and ever) to actually be based on a specific location from Fire Emblem. The other stages are just a generic castle and a generic colosseum. It's also one of the few stages in Smash 3DS not to have any new music, with id (Purpose) being directly taken from Fire Emblem Awakening, and the Melee Fire Emblem song from Temple. They're still both great songs, so nobody's complaining. Actually, we take that back. ID~Purpose was shortened from eight minutes to two, making it one of the many songs to be completely butchered, along with other masterpieces as the DK Rap, Engage the Enemy, and the Sonic vocals.