Before I get into any of this, this is going to be HUGE spoilers so be advised!
For those who have decided to keep reading this you are here to find how to get the secret ending. It is not hard but once you scatter the ashes of Faye the wife of Kratos, and mother of Atreus you must walk back down the stairs you came up. Listening to an important conversation between Kratos and his son, you venture back into the travel room.
Once you get in there travel to Midgard upon your arrival you will hear another conversation, this time between the 2 dwarven brother. After this you can take the door directly behind you if you are facing the dwarves that you can use to fast travel back home.
As soon as you get home you will be able to enter the house which will prompt you to sleep! Now you will wake to a new cut scene, and you will acquire the “secret” or possibly “true” ending to God of War!
Let me know your thought on what this ending means, or if you think it is a hint to what is to come for God of War.