what is the note book and what are the requirements?

Got the note book by killing all the vanilla bosses and combine them by talking to the jeweler I am now left with an item called the note book and can be consumed if the requirement are right. These are my hypothesise

Dark sun - does it mean solar elcipse, the moon or a new event tho it does not show on the boss checklist mod.
Location - where is the portal between the 2 world. Is it in the air, underground the borders of our world.