After her grandmother passed away, you, who has no other relatives, inherits an old mansion.

One thing that bothers her is the staircase leading to the basement. At the bottom stands a heavy iron door, secured with a rusty padlock. Even after searching the entire mansion, the key is nowhere to be found.

Strangely, her grandmother's diary contains no mention of that door at all. Only on the very last page, a single line remains.

[Never, under any circumstances, open the basement.]