From Bulbapedia, the community-driven Pokémon encyclopedia.
A Monster house from the games Red and Blue Rescue Team.
A Monster house from the game Gates to Infinity.
A monster house (Japanese: モンスターハウス Monster house) is a special type of room in some dungeons of Pokémon Mystery Dungeon. Monster houses will usually contain many items (typically Gummis or Wonder Tiles) and they are usually very spacious. Once the player enters the room, many Pokémon in large numbers will drop down from the roof and start to gang up and attack the player. At this point, the music will change into a more menacing tone. The Pokémon inside are always ones that can be found in the dungeon it is located in. A monster house will also increase chances of a rare Pokémon to appear. Monster houses only appear in later dungeons. In Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team, they begin to appear in Uproar Forest and in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness and Explorers of Sky, they begin to appear in Amp Plains, although they can appear earlier when players are doing a job. Other than only appearing in some dungeons, there are no restrictions on where a monster house can be located. They can even occur in the room the player spawns in and the room with the stairs to the next floor. They are featured yet again in Pokémon Mystery Dungeon: Gates to Infinity and begin to appear in Kilionea Road.
The player will realize that they are in a monster house when the music changes, Pokémon suddenly pop up all together, and the screen displays the text "It's a monster house!". If the player tries to escape, the Pokémon of the monster house will follow until they are all defeated. Another way that a player may realize that they are about to enter a monster house is when their partner Pokémon stops following them. One of the skills that all IQ groups except C, D, and G have is the ability House Avoider, causing them to avoid monster houses.