What does the Infinity Blade unlock in the dungeon?
Solution 1:
When you have the Infinity Blade equipped and you "examine" the sword shaped lock in the centre of the dungeon, you place the sword inside and it descends into the floor. This unlocks three more doors in the dungeon, each of which leads to a challenging boss fight with one of The Deathless Kings: Ealoseum, Gortoel and Kuero. Once you've defeated each of these bosses, it unlocks a final showdown with Zero Mech and an alternative ending if you prove victorious.