From Dragon Mania Legends (DML) Wiki
The Sacred Apple is a magical apple and one of the rarest treasures in Dragolandia. Feeding a Sacred Apple to a Dragon increases the maximum obtainable level of that Dragon by one, a process referred to as overcharging.
Feeding a dragon a sacred apple does not immediately raise its level, the dragon must still be given Food level up.
A maximum of 5 Sacred Apples can be given to the same Dragon, raising its level to level 125. Once the Dragon's level is overcharged, the in-game text that indicates its level changes to the color teal in all parts of the game interface; this change indicates at a glance that the dragon is overcharged, setting it apart from other Dragons.
Sacred Apples can be gathered from Events as special prizes.