The mansion of younger sister

The sister's mansion is a "secret" location mentioned only in the book 'Strange Diary'. The book has been translated in the English version, but instructions are marked on the user created world map on how to find it.

There are a great number of small medals here for such a small area. There is a guaranteed book of Rachel No.4 here. Silver cats and young ladies are here, but cannot be charmed. A younger sister sells sister's lunches here which reduce Sanity by 30 when eaten.

It is also treated as a unique dungeon like Lesimas (layout and items do not refresh), and adventurers will sometimes find artifacts here. You can also go back to it using a scroll of return at any time after you've discovered it.

The younger sister can summon other younger sisters when it attacks something, so inducing this by monster summoning or antagonizing her yourself and returning will summon a large number of level 1 NPCs to perform for. Do it indoors to prevent interruption by snowballs (alternatively you can do it outside if you kill the young ladies, they'll respawn later) and be sure to clear a space around you for rewards to land. Note that the shopkeeper herself is level 50, so you'll want to remove her via a rod of teleport or spell of teleport other.