Although it sounds a little like the Norse idea of Ragnarök, shortly after Valheim's launch some players discovered a serious "world-destroyer" bug that could reset your progress - a problem that was very much not intended. Thankfully, developer Iron Gate Studio has today rolled out a patch to address that, so now the main bug you need to worry about is the Deathsquito.
As reported by players at the time, it seems the world destroyer bug was indeed connected to logging out and pressing ALT+F4 at the same time. Following the patch, this will no longer cause the end of days.
Elsewhere in the patch notes, Iron Gate explains the biggest change is the "updated socketed backend" which should help improve connection issues. Vulkan support has also been added for Windows users to help with some crashes related to GPU drivers, and can be enabled by adding "-force-vulkan" as a launch option in the Steam properties menu for Valheim.