The Churning Rift of the World is a 30-fight boss rush found in the middle of the world map for the character of your choice once you've completed Armageddon once. Whichever character you choose to play as, gear them up with plenty of helpful potions, materials, and equipment that you will need, as there are only two rest areas in the lengthy chain and they offer overpriced items. There are no checkpoints, and if you decide to leave, you'll need to start back at the beginning. I would say bring the character that you raised up to Level 65 for this, as they will have all of the skills and perhaps have some decent upgrades to their preferred skills.
The bosses are:
Paladin Knight
Belial
Plant Monster
Velvet
Bugbear
Wagner
Phoenix Rider
Leventhan
Boltergeist
Queen Odette
Myconid
Mercedes
* Rest Area (after floor 12)
Manticore
Lord Brigan
Dwarven Battleship
Halja Wrath
Axe Knight
Skuldi
Child of Fire Vulcan
Oswald
Barbarian Warrior
King Odin
* Rest Area again, I believe)
King Ooze
Beldor + Belial
Silver Titan Frost Ben
Beast of Darkova
Lord of the Netherworld Gallon
Inferno King Onyx
Cauldron
Leventhan, King of Snakes