If it's still actual.
Extra boss unlocked after beating final boss, Veilac.
To reach Horror world you must switch game language to korean and you will have a new building in cosmos square - here the entrance.
There is no eng translations but you still can beat it randomly somehow (I did act 1, and left act 2 because stuck for too long).
It's do not apply to HW bosses, 1-2 have fixed patterns, 3rd have some random, but you can memorise which atacks korean hieroglyphs at the bottom means so it's not a problem too.