Northri Stronghold #1
Once you reach the Mystic Gateway and Sindri's Shop, turn around to face the boat and the Raven will be perched above.
Northri Stronghold #2
Once you've taken your boat past the Reaver Ship after you defeat the Traveller, dock at the next stop where you need to use the chain to raise the gate. Head back along the platforms and break open the boards to find a coffin. Look into the air and to the left from this position to find the last Raven here.