From Bulbapedia, the community-driven Pokémon encyclopedia.
The Relic Stone (Japanese: 聖なる森のほこら Sacred Forest's Shrine), found in Pokémon Colosseum and Pokémon XD: Gale of Darkness, is a shrine located in the Relic Forest of Agate Village. The Relic Stone has the power to purify Shadow Pokémon once their Heart Gauge has gone down to zero.
It is said to be the place where Celebi once landed, and so had the power to let Shadow Pokémon remember their happy and carefree lives before the doors to their hearts had been closed. Visiting the Stone would let them relive their memories and revert to normal Pokémon once again.
If the Time Flute (found only in Pokémon Colosseum) is used at the Relic Stone, Celebi appears and purifies one Pokémon, regardless of level and Heart Gauge.