The Seven Treasures and their Guardians
The Seven Treasures (Japanese: ７つのひほう Seven Treasures) are items in Pokémon Mystery Dungeon: Explorers of Time, Explorers of Darkness and Explorers of Sky given to players after they defeat the Guardians in seven secret dungeons. They are all based on musical instruments.
The only ways in which players can access these dungeons is by being granted the "Secret Rank" by rescuing Scizor from Froslass in Crevice Cave, and then entering the correct Wonder Mail codes or downloading the missions via Nintendo Wi-Fi Connection. Due to this, the missions can actually be obtained twice. In Explorers of Sky however, the mail can be obtained in Spinda's Café.
Before players fight one of these secret bosses, it will address their team as the "Seekers of the Seven Treasures!" and then challenge them. Upon the defeat of a boss, the player will obtain its respective Treasure. After winning them from the bosses, the Seven Treasures may be sold, unlike many other rare items in the game. The bosses may also join the player's team. After the recruitment of a boss, additional copies of its respective Treasure are obtained whenever the player reaches the end of that boss' dungeon. The player cannot reach the interior of these dungeons (where the treasure and bosses are) while completing missions inside the dungeon.
Each of the treasures increases the recruitment rate by 20% for Pokémon of its respective type. Grass Cornet increases the recruit rate for Grass types, the Fiery Drum for Fire types, and so on.
If one of the guardians wants to be recruited upon its defeat, it will reappear after the player retrieves the treasure:
"Oh? The Pokémon is saying something. It seemed to develop a kinship with the player while battling! It says that it wants to join the team!"