From https://valheim.fandom.com/wiki/Events:

"Every 46 minutes there is a 20% chance for a random event;

Available events are based on which enemies have been defeated in the game world.
Most events require defeating a boss to become available (see Enabled by column)
Some events are permanently disabled after defeating a boss (see Disabled by column)
For each event, check that any of the players fulfills the following conditions:
The player is in the required biome (see Biome column).
The player is not in a dungeon.
The player is within 40 meters of at least 3 base structures. This is not required for the "You are being hunted..." event."