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If you are playing on the web, you cannot open Valhalla from any god map or from the Father map.
Go back to the Map Manager, and click on the "Main" button. You should be able to get to Valhalla from there.
It is hard for us to know what the 19th quest is. There are 17 main Frigga quests and a Beowulf side quest with 4 locations that becomes available after the 6th Frigga quest. We don't know in what order you opened them.
But we can be fairly certain that you either need to talk to:
- Frigga in Valhalla, or
- Beowulf, who is either in Fjordheim, or, after Dragon's Domain, Heorot Hall
https://wiki.diggysadventure.com/index.php?title=Scandinavia_(Quests)
