An Idunn Apple is used as a way to increase Kratos’ health. This game can be hard at times so having the max amount of health. For every three apples you find, you will get a fairly substantial increase to your overall health. There are a total of nine apples, meaning you can upgrade your health bar three times. Once you obtain the final apple you will unlock the “Idunn’s Orchard” trophy.
All of the Idunn Apples are located inside sealed chests that require you to interact with three runes that will be hidden in the area. The runes will match the symbols on the chest. In most cases this will involve destroying three black jars in the area that contain one of each of the runes on them. Another frequent one involves hitting three bells before the first one stops ringing. Finding all three runes can be tricky at times, but they are never too far from the chests.
If you are having trouble finding the chests or the rune locations needed to open the chests up, check out the video guide below and this will show you where to find all of the Idunn Apples in God of War.