GW2 Forgotten Stream achievement guide with detailed walkthrough. Forgotten Stream is a new achievement added with the Halloween patch and it is located in Cursed Shores.
Behind the Cathedral of Verdance is a is a river and there is a semi hidden path you can go through, The door at the end of the path is locked there are plenty of fast respawning risen along the way. Bring a friend if you don’t feel confident soloing all the risens. No chest here but you will get the Forgotten Stream achievement near the locked door.
See Halloween Patch Explorer achievements guide for other achievements.