Trying to figure out how to open the gate to Northri Stronghold in God of War? This is a hidden area you might stumble across once you reach the Lake of Nine. You don't go there as part of the main story, so, like Veithurgard, if you're not on the hunt for treasure maps or other collectables, it's easy to miss.
If you do happen to stumble into the area by chance, you'll find your way barred by a locked gate and a mechanism used to open it. There are no clues about what you have to do to access the area, so I'll save you some frustration and help you out. There are some mild spoilers ahead, so click away now if you'd rather not take the chance. Here's how to open the Northri Stronghold gate in God of War.
God of War Fafnir's Hoard: How to get the Northri entry stone
You need to complete Fafnir's Hoard, one of Sindri's Favors—or side quests if you prefer—to get the entry stone required for the mechanism on the gate of Northri Stronghold. This favor becomes available when you return to Midgard after completing The Light of Alfheim story quest. Speak to Sindri at any of his shops to pick up the Fafnir's Hoard favor.
The quest marker takes you over to the west of the Lake of Nine. You can get there by boat by sailing under the ribs sticking out of the water there. Travel up Fafnir's Ravine until you can dock, then take the lift up. Enter the cave and follow the path until you reach a blue crystal. Have Atreus shoot it with an arrow to make a light bridge and cross the other side to find one of Sindri's shops and a Mythic Gateway.
Continue until you reach a wooded area, deal with the enemies here, and head into the tunnel behind the waterfall. Once you get to the large cavern with a pedestal on a circular platform, use the entry stone on the pedestal.
Use the newly made bridge to access the room beyond. You'll have to deal with a couple of enemies, then pick up the entry stone and head back to the pedestal. Repeat this three more times with the different rooms until you unlock the path to the door that leads into a large sunlit chamber.
Speak to the Reaver spirit at the back of the room to retrieve the dagger, then defeat the two ogres that show up. Now it's time to head back to Sindri to complete the quest. Speak to him again and accept the following favor, Family Business. If he doesn't immediately offer you another quest, you'll need first to finish the 'A New Destination' story mission.
How to get into Northri Stronghold
You can reach the entrance to Northri Gorge at the northwest corner of the Lake of Nine. Follow the river until you reach a gate blocking the way and a dock to your right. There's a Mythic Gateway here and a mechanism to open the gate.
Assuming you've completed the Fafnir's Hoard favor and spoken to Sindri to initiate the Family Business quest, you should have the entry stone in your possession. Use it to open the gate, then hop back into your boat to explore Northri Stronghold.