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The Relic Fragment (Japanese: いせきのかけら Remain Fragment) is an object in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness and Pokémon Mystery Dungeon: Explorers of Sky. It is owned by the player's partner, who has had it for as long as they can remember.
Team Skull takes it twice over the course of the game. The first time is at the very start when the player and partner first meet Zubat and Koffing, the other being when Team Skull realizes the Relic Fragment needs to be used in order to get to Hidden Land. The first time, the player and partner go through Beach Cave, their first dungeon together and battle Zubat and Koffing to get it back. The next time, Team Skull gives it back when they are badly beaten up, right before Kabutops and Omastar are faced in Brine Cave.
The Relic Fragment only appears in front of good-hearted Pokémon. When taken to the bottom of Brine Cave, it interacts with a similar pattern on the wall, signaling Lapras to take the Pokémon to the Hidden Land. It must also be inserted into a small hole, in which it fits perfectly, to activate the Rainbow Stoneship, the mechanism for accessing Temporal Tower from the Hidden Land. In this way, it acts as a key to the Hidden Land, preventing all but those with the purest hearts from gaining access to Temporal Tower.