From the Super Mario Wiki
Coliseum is a stage appearing in Super Smash Bros. for Wii U. The stage is based not upon a specific game, but rather upon the recurring arenas in the Fire Emblem series. As in Arena Ferox in Super Smash Bros. for Nintendo 3DS the stage is set inside the arena itself, though by default, it is a flat, grounded stage. During the match, parts of the floor rise to create platforms of varying heights.
The Omega variant of this stage is set on an elevated platform over a pit.
The unlock match for Lucina takes place here.
Coliseum reappears as a stage in Super Smash Bros. Ultimate.
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