Castlevania Advance Collection

In order to unlock this fight, you need to fight and beat Julius. In order to do that, you need to fight Graham with 3 specific souls equipped. The first is the flame demon and can be found in the Forbidden area next to the underground graveyard. The second is the Giant Bat soul in The Arena after the Balome fight. The third is the Succubus soul which can either be found in one room in The Arena or several in the Top Floor before the Graham fight. All 3 must be equipped when Graham is defeated. You will then absorb the Dracula soul and can pass the dark door in the Floating Garden. Julius is weak to dark or unholy effects but may require a lot of healing items so prepare accordingly.

Once Julius is defeated, you will be allowed to enter the Chaos which is sort of like a twisted Clocktower. There are maps for it online but no map in game, so be aware of that. Once you reach the end, you will face Chaos. There are several different things going on at once on screen that can damage you so bring plenty of healing items. Destroying the 4 eyes around the perimeter of the room can weaken the orb of chaos in the middle or you can just try to destroy the orb outright. Either way, beating this boss unlocks the best ending and the achievement.