Valkyries are female warriors that serve the gods. They are generally considered good, but due to some form of corruption they have turned evil. There are nine of these hidden around the land and you must fight all of them to complete the side quest. Killing all 9 of the Valkyrie in God of War will unlock the “Chooser of the Slain” trophy.
The Valkyrie battles are generally found in Odin’s secret chambers, with there being a few exceptions to the case. These are usually quite easy to find, but you do not possess the means to unlock these doors until later on in the game. Keep an eye out for doors that are locked with a mirror seal. The BOY! will often say that these doors are locked with magic when you try to interact with one. Once you possess the means to unlock the door you will know as the game will guide you through the process of unlocking the seal.
Once you have the means to unlock the doors to the Valkyrie battles you should not rush into them. These battles are incredibly difficult. Even if you have gotten familiar with the combat in the game, you will struggle to get through these fights without taking a beating. It is best to wait until you have maxed out all weapon skill trees and have some high powered armor and stats. There is nothing stopping you from going to fight them whenever you like. Some are more easier than others so you may manage to kill a few of them early on.
The video guide below will show you the location of every Valkyrie in the game. Defeating them may be something you have to come back to at a later date due to the difficulty. Thankfully there is a gateway at the entrance to each one of them. If you find you are getting your ass handed to you, you can come back once you are stronger and show her what’s what!