After the conversation with Mr. Rich, the Rolling Stone will appear. As soon as the 'touching this will result in a Game Over' message appears, hold right and dash outside. The Rolling Stone has a possibility of spawning in any room of the mansion, including the outdoor passageways - as such, it is recommended that you save very often during this level.
There are also a couple 'trick spots' that will cause the Stone to appear and kill you instantly in you inspect them. One is the writing on the wall by the bathroom on the 1st floor west wing, and another is under the sink in the kitchen by the library.
Don't approve of him, but gotta trust him
The first room you enter upon escaping Rolling Stone is the bar. Once you enter, you'll encounter Hol Horse, who is fending off zombies. He'll reveal that he was trapped in the mansion while searching for a partner, and suggest a temporary ceasefire while you escape the mansion. Hol Horse will join the party for the duration of this stage. He can attack twice per turn, has a high hit rate, a high chance of inflicting 1HKO, and healing techniques.
If you're playing as a girl, he'll cry tears of joy that you're alive and that he no longer must forfeit the title of World's Nicest Man to Women.
Escaping the Mansion
To leave the mansion, you have to unlock the underground passageways to the submarine and activate the submarine's power supply. There are three passageways - one to the submarine itself, one to the keypad that powers on the submarine, and another that allows access to Charlie's room under the room, which contains a computer that enables use of the aforementioned keypad. The answers to the security questions can all be found in the library in the east wing.
For a more detailed step-by-step guide with all security codes and a map, see the full walkthrough.
On the 1st floor of the west wing, where the computer is, there's a bathroom you can enter. This is a safe room where the stone cannot enter. If you run out of recovery items and can't progress, you can get infinite Bottled Water from the sink. You can also chat with Hol Horse or read the bookshelves for clues on how to progress.
Requirements: Current day is 21 or higher
In the kitchen near the library in the east wing, there's a cellar that you can enter. This is another safe-room where the stone cannot spawn. If your current day is under 26, you can also use the beds to rest, and you'll get a short cut-scene with Hol Horse and your other party members. These scenes have a lot of random variations, and are different depending on your gender and which two party members accompanied you in Bengal. If you type in the second password and haven't already been to the cellar than the cellar door will be locked.
Requirements: Day 20 or under, have not entered to code to unlock Charlie's room
A word of warning: This sidequest contains some very tough bosses. If your level is too low to defeat them, there will be no way to progress the story other than grinding using zombies until they're beatable. As such, it's recommended that you make a separate save before starting it.
If you enter the Cellar above on day 20 or lower, a short cutscene will occur, and you'll encounter a young woman who was also trapped in the mansion. If you keep checking back here after your encounters with the mysterious attacker who uses Power of Dreams, her dialogue will change. Finally, after you activate the submarine, come back to pick her up for a final cutscene and a battle, which will reveal some backstory about the people trapped in the mansion and the nature of the Slaves to Fate.
In the upstairs room containing the computer that allows you to power on the submarine, a man named Silver Fox is hiding from Rolling Stone. Talk to him, and he'll plead for mercy - you can choose to help him, but Hol Horse will shoot him anyway. If you choose not to help him, you'll get 3 bad karma and anywhere from 5000~30,000G as well as Rations x10. After he's dead, the Rolling Stone will appear, moving at a much faster rate. You can actually progress without talking to Silver Fox at all, so this event is completely optional.
There's an additional panel in the lower left corner of the room. Enter "help", and a hole will open in the floor, allowing you to quickly escape Rolling Stone. It will also drop a ladder to the attic, which contains useful money and items, but be careful! Rolling Stone will already be in there, and the room is quite small, making dodging it difficult.