The special mission is Inphyys final mission against the king of nights. To unlock this secret mission, you must beat the game with every single character then load up Inphyys file. Once that has been done, look under Ywa-Ue-Uar and there should be a mission called Another World.
Beating the King of Nights can be fairly difficult if you dont know what youre doing. First things first, if you cant beat it, keep killing yourself and eventually the difficulty will be lowered. If you look at the loading screen it should show Mission Name Level # (the # states the difficulty. Lower the number the easier it is). Now, what you should do is make sure Inphyy is level 9 because you need the Seraph Butterfly move. After youve become level 9, start the mission and zip through the middle using dash. Dont stop to fight the army of night or branch off and do other things. Theres no point as it would consume too much time. If you have time to spare might as well do it. Once youve reached the king of nights, you will see Goblins and Trolls fighting him with an insane amount of army of night soldiers surrounding him. This is definitely a good thing (because of red orbs). What you want to do is make sure that your orb gauge is full, then pull off Seraphs butterfly. You have to make sure hes standing on the ground and not flying because if hes flying than it wont work at ALL. It could take anywhere from 3-7 hits to finish him off. If youre lucky youll find the armband that allows for no orb consumption while using deadly attacks. Also, make sure if you see those silver balls flying at you, Dash and dont stop because they hurt you real bad. Keep Seraph Butterflying while the king of nights is on the ground and you should finish in no time. Its not a difficult task at all once you get the hang of seraphs butterfly. You can also use your orb spark and itll do a very nice amount of damage to him.