From Bulbapedia, the community-driven Pokémon encyclopedia.
The dais underwater with the grail
The Rainbow Grail (Japanese:にじのせいはい Rainbow Grail) is found in Pokémon Ranger: Guardian Signs and is an ancient artifact from the Oblivia region. It can be used in times of crisis to call the legendary bird Ho-Oh.
The Rainbow Grail is said to have helped save the world from peril with the help of the Hero of Oblivia. Since then, it has been a custom of Cocona Village that each year the villagers get to take part in a trial, and the winner is given the honor of holding the Rainbow Grail over the Rainbow Dais. However, their Rainbow Grail is merely a copy made of paper-mâché. After Dolce Island is obliterated, the player must take the Rainbow Grail to the Rainbow Dais. Once the real Rainbow Grail and Rainbow Dais are found under the ocean . The player then takes the Rainbow Grail to the Rainbow Dais and the Rainbow Dais then rises out of the ocean and the player is given the sign used to call Ho-Oh. After Ho-Oh is befriended, it will destroy the barrier on the Sky Fortress.
After the game's events Blue Eyes and her platoon are rescued from a rampaging Lugia after attempting to steal the Rainbow Grail from the bottom of the sea. However, this Rainbow Grail too is merely a cheap fake and wasn't of any value.