Get to Chapter 5 first called Golden Child Crimson Blood and head to Flaming Mountains. Youll need to deal with those five element carts which are pretty tough but worth it since they unlock the cave.

Start with Brown-Iron Cart in Woods of Ember, then grab Gray-Bronze Cart nearby at Height of Ember. Next up is Crimson Silver-Cart in Furnace Valley, and dont forget the Rusty-Gold one at Cooling Slope in Field of Fire.

After beating each cart, check in with Pale-Axe Stalwart, he's actually important for this whole thing. Once you've dealt with all those carts, go back to him one last time, and he'll point you to where you need to go.

Look for a mirror near the cave entrance, break it, and youll find your way in. The cave's got some nasty bosses waiting inside, but theres good stuff in there if youre trying to get the true ending. Good to know that having some upgraded abilities before heading in makes things way easier, especially for the later fights.