Welcome to The Secret Dungeon!
to those who haven't unlocked it, here's how you can access it:
Finish the game once(REMEMBER YOU WILL LOSE ALL YOUR ITEM, Exclude Money & Grow Book)
Continue the game until you reach the Old Schoolhouse where you fight Chimera(Without Vayne)
Reach the end of the road, at the edge wala! its now open!
Here is where the monster power is enhanced to a MEGA HARD level
You might be wondering how to beat them, it seemed impossible though but remember, POISON!
so You might want to have a venomous Weapon +Lots of Unis Etc that poisons foe. Remember, it's harder than u thought so try to just skip them. I wrote this to those who want to complete the monster collection

Another Reminder:

-At The end you will encounter a dark ball-like thing, if u choose to fight, u'll meet Dalvero (Alterier Iris 3 character), and it goes on and on until Darkash. BUT! there are rewards for defeating them :) so don't just play the boring way, a bit struggle is always fun