Sacred Ash
The Sacred Ash is an item in Generations II, III, and IV. When used, it revives all of the Pokémon in a trainer's party with full HP.
Location
Generation II
In Generation II, when the player battles Ho-Oh at the Tin Tower, it will be holding the Sacred Ash.
Generation III
In Generation III, the Sacred Ash can only be obtained in Pokémon FireRed, LeafGreen, and Emerald. Once the player defeats or catches Ho-Oh on Naval Rock, he/she can use the Itemfinder while standing on the spot where Ho-Oh was to find the Sacred Ash. The Sacred Ash can also be obtained in Pokémon XD as a prize for defeating all 50 Battle Disks.
Generation IV
The Sacred Ash can't be found in Generation IV. The only way to obtain it is to have a Pokémon hold the Sacred Ash in a Generation III game and transfer it over via Pal Park.