R'lyeh (Bermuda Triangle Lair)

 is a limited-time premium building that was released on September 25, 2019, as part of Cthulhu's Revenge 2019 Event. Upon completion, M'nthster is unlocked.

It can be obtained through the R'lyeh (Bermuda Triangle Lair) Bundle if the player doesn't have the Sunken Temple.

Regular
M'nthster
 * Clean House - 24h

Quest Based
M'nthster
 * Unpack Bachelor Pad - 4h

Trivia

 * It can only be placed on the ocean.
 * It only appears when M'nthster is sent on his regular job or quest job. When he isn't sent on the job, it remains as a whirlpool in the ocean until he gets sent on the job.