Vibrating square
The vibrating square is the square where the invocation ritual is to be performed for entry into Moloch's Sanctum. It is always located on the penultimate dungeon level of Gehennom, which could be between levels 44 and 52. It is the lowest level that can be reached without performing the ritual; until the ritual is performed, there is no staircase leading down from that level. Only the Sanctum level has a higher dungeon level number.
The level with the vibrating square is in most other respects an ordinary maze level of Gehennom.
Identification
The only way to locate the vibrating square is to move over it, at which point you will get the message: "You feel a strange vibration under your <feet>.", or "You feel a strange vibration beneath you" if you are flying or levitating. This message will always occur if you walk on top of the vibrating square, but not if you jump over it or are carried over it by an engulfing monster. There are no other special features of the square.
The vibrating square cannot appear near the edge of the screen nor the staircase, because there must always be room for the ring of fire traps and water to appear around it.[1] Specifically, it will be at least five squares from the left and right sides of the map, at least four squares from the top or bottom, and not within a 23x23 square area centered on the upstair. It will also not be in a straight line with the upstair or on the same square as a trap, although it may be on the same square as a boulder or other item. It helps to mark ineligible places (and where you have searched) on a screen shot in an image editor.
Many players will mark the vibrating square by leaving an object on it once they have located it. This is one of the leading uses for the mostly useless iron chain, but it is hardly necessary to find one for this purpose. Any object that most monsters will not want to pick up, or is protected by an engraved "Elbereth," will suffice, and the vibrating square itself never changes location. The general location of the square can also be permanently marked with a scroll of light, wand of light, or the spell from a spellbook of light. Some players polymorph into a xorn so they can traverse the level line by line, or into an air elemental to get it done fast.
This MSGTYPE setting makes it harder to accidentally miss the vibrating square if you are using the tty interface:
MSGTYPE=stop "You feel a strange vibration under your *." MSGTYPE=stop "You feel a strange vibration beneath you."
In this example map, the eligible places are marked with (X). Red spots are too close to the edge, blue ones in line with the stairs, and yellow ones too close to the upstairs.
|.......|...............|.......................|.|...|.......|.........|...|
|.-----.|.-----------.---.-----.-----.---.-----.|.|.|.|.|.---.|.|.-----.---.|
|.|.....|...|.......|.....|...|...|...|.|.|...|...|.|...|.|.|.|.|...|.|...|.|
|.---X---X---X-----X---X---X|X-----X--|.|.|.|.|--.|.----|.|X|X|X|--X|X---.|.|
|...|X|XXX|XXX|X|X|XXX|X|XXX|XXXXXXX|.|...|.|.|...|...|.|.|XXX|X|X|X|XXX....|
|.|.---X---X---X|X---X|X|X-----------.|.---.|.-------.|.|.|----X|X|X-------.|
|.|.....|...|...|...|.|.|.|.......|.....|...|..<|.....|.|.|.....|.|.......|.|
|-------|X---X---X--|X--|X|X-----X|X-----.-----.|.-----.|.|X-----X|------.--|
|...|XXX|X|XXX|XXXXX|XXX|XXXXXXX|X|X|.....|.|...|.......|..X|XXXXX|X|XXX|...|
|.|.|X|X|X|X-------X---X|------X|X|--.-----.|.---------.-----X---X|X|X|X---.|
|.|..X|XXX|X|XXXXX|XXX|X|XXXXX|X|X|X..|.....|.............|XXXXX|XXX|X|X....|
|.|------X|X---X|X|X|X|X|X---X|X|X|X|.|.---.-------------.-----X|--X|X|----.|
|.|..XXXXX|XXX|X|XXX|X|XXX|X|X|X|XXX|...|.....|.......|.|..XXXXX|XXX|X|X..|.|
|.-----------X|X|----X-----X|X|----X|--------.|.-----.|.|.-----X|----X|X|.|.|
|...|.......|.|.|.......|...|.|...|.|.....|...|...|...|...|...|.|.....|.|.|.|
|.|.|.---.---.|.---------.|.|.|.|.---.---.-------.|.-------.|.---.-----.---.|
|.|.....|.....|...........|.....|.....|...........|.........|.....|.........|
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