This stage unlocks after beating every single stage of every single character in the game. It opens up a final stage for the Inphyy character.
It's not worth attempting this stage unless you are level 9 already, or have over 190,000 points in which case you will level up to level 9 before reaching the boss of the stage.
* Seraph butterfly attack is only available at level 9 and it drains your orb guage (jump with A and then press A and then Y when in mid-air).
* The deadly armband is key to this tactic as this prevents your orb gauge from draining.
* Only attack the overlord when he is on the ground using the Seraph Butterfly attack, when he is in the air the attack appears to cause no damage.
I only obtained the Deadly Armband on this stage, I had to equip Clover Leaf 2 along with the Divine Tiara and after a few unsuccesful plays through the level eventually one of the Creatures of the Night dropped the Deadly Armband.
Personally, I had equipped the Level 7 Prominence blade, the Knight's Emblem, Life Globe 3, Deadly Ring, Divine Tiara and Deadly Armband and managed to beat the overlord first go once obtaining the armband.
As a note there are 4 sub-areas on the map, don't bother with the top-right area as there are some seargent level baddies which take a long time to kill, I only took out the two left-side areas and still obtained a A grade, without even scoring a combo much above 300.