From Dragon Mania Legends (DML) Wiki
The Sacred Apple is one of the rarest treasures in Dragolandia.
***Information contained herein may be incorrect or incomplete and is subject to change.***
Feeding a Sacred Apple to a maximum level Dragon unlocks one additional level for this Dragon and allows it to be fed again and thus overcharge it.
A maximum of 5 Sacred Apples can be given to the same Dragon.
Sacred Apples can be gathered from Events as special prizes.