This is for Completing the Bonus Dungeon. Once you defeat Artorious and Innominat, your game will pick up right before the final fight. Leave and go just outside the Empyrean's Throne and speak with the Katz there. It will speak of some tragedy you caused and tell you to go to Mount Killaraus. Go to the Area where the Katz chest was and directly across from it will be the Earthpulse Rift that leads to the Bonus Dungeon.
The enemies range from Lvl 70 - 124 throughout the dungeon. It is recommended you attempt this in a New Game+ with 2x or 3x EXP but it can be done in your first play-through with some intense frustration and grinding. I had to be Lvl 98 to complete this. You will just need to progress as far as you can and grind where you are having issues.
The dungeon itself is a pain in the butt because of how it is designed. There is usually 3 rooms of puzzles and then a room with a big flame of malevolence. Enter it and defeat the waves of enemies. Upon defeating it, a timer will start and you will need to find tiny malevolence flames and defeat the wave of enemies with in them all before the timer ends. If the timer hits 0 it removes the small flames and the big flame reignites. If you complete the final flame after the timer hits 0, it still registers as a fail.
There is a Katz in the area that can make things easier for you but it will cost Katz souls. I recommend taking advantage of it to locate the small flames and extend the timer. These must be purchased before entering the giant flame, the purchase is also permanent per level. Once you complete that you can continue on to the Boss of the Floor. Defeat the Boss and the next room will contain another Katz. Speak with him and use 100 souls to create a warp point so that when you fail you can teleport there, rather than starting over from the start.
You will have to do this 6-7 times. This is also required to obtain the Dual Mystic Artes. 3 of the Bosses upon their defeat unlock a dual Mystic art. They are: Velvet\Laphicet, Eizen\Rokurou and Eleanor\Magilou