Yeah, there are spoilers here. Pretty big ones. Be warned.
God of War doesn’t end when you finish the main story. Credits roll after you complete the final battle, but they’re superimposed over the shoulders of Kratos and Atreus as they finish their journey to the highest mountain in the Nine Realms. After that it lets you immediately return to Midgard, where you can complete whatever sidequests are still available, and where you can now access two new realms, Niflheim and Muspelheim, which are basically two different variations on combat arenas. You could easily spend twice as much time exploring every part of this game as you do playing through the main story.
One of the secrets hidden in the game that’s only accessible after finishing the story is a second ending. It’s short but it introduces a pivotal character who’s mentioned often in the game but never actually seen, and hints at a crucial conflict in the inevitable sequel. Here’s how to see it.
After spreading the ashes of Kratos’s wife over Jotunheim, you can go anywhere you want in the Nine Realms. If you head back to Kratos’s home, which is easily accessible by fast traveling through any Mystic Gateway, you’ll be able to sleep in Kratos’s bed. If you do that, you’ll trigger that secret ending. We won’t completely spoil it for you, but it’s set some years after the game (although Atreus doesn’t seem to have grown at all…) and it sees a very angry visitor storming his way towards Kratos’s house. After a quick flash of a famous weapon from Norse mythology it fades to black, and the full credits for the game finally run.
If you’ve already finished the story and kept questing through Midgard and the other realms, don’t worry: you can access this ending at any point. Just head to that house, take a Rip Van Winkle nap, and then get startled awake by somebody with a pretty good reason to hate Kratos. And if you can’t wait ‘til you get to the story’s end, you can probably just cheat and head over to YouTube and watch the whole scene right now. Cutting corners is what the internet’s for, after all.