When I just got Arokh I flew around Surdana and at a certain point I reached this place full of cannons, white dragons, and this giant castle, I died immidietaly, but what is that place?
That is the entrance to the city of the dammned. Its a sidequest which only becomes available after you have completed the monastry sidequest and spoken to Jade afterwards. I won't tell you anymore as it will spoil the game for you but if you are still stuck post back here.
Speak to the people in the tavern and it should show up on your map. The best thing to do is explore everywhere and talk to everyone you meet to be able to get sidequests.