The Sacred Ash is a healing item found in all games since Generation II. Unlike other items in general, which work on only one Pokémon, the Sacred Ash, when used, revives all Pokémon in a Trainer's party to full HP and PP. It is very rare, and can only be used once, making it a very popular item for cloning glitches.
The Sacred Ash is typically found only once per game, and will often have some relation to Ho-Oh.
Another Sacred Ash can be found in the Battle Pyramid; however, it cannot be kept.
Currently, the only way to get a Sacred Ash in Pokémon Diamond, Pearl, and Platinum is by having a Pokémon hold it in Generation III and sending it through Pal Park. Wild Ho-Oh, which can only be encountered with a cheating device, will always be holding this item.
In Pokémon HeartGold and SoulSilver, the item is held by The ho-oh found on top of tin tower, like in Generation II