The Ruined Hall
From the Super Mario Wiki
The only objective in this stage is to defeat the boss, Galleom, who was first fought by Marth, Meta Knight and Ike above ground previously, right near the underground Ruined Hall. However, Galleom was not completely destroyed, but instead jumped; because of its immense weight, the ground crumbled to pieces, and Galleom fell through into the Hall as Lucas and the Trainer entered, forcing them to finish the boss themselves.
After Lucas and the Trainer Galleom defeat it, Galleom reveales that a Subspace Bomb is attached to it, and then grabs Lucas and the Pokémon Trainer in order to detonate the bomb and take them with it. Luckily, they are saved by Marth, Ike, and Meta Knight. After Galleom is destroyed, the Pokémon Trainer and Lucas join the swordsmen.
Stage Clear Reward