Events have a 20% chance of starting every 46 minutes if there is a player within 40 meters of at least 3 base structures (except for the "You are being hunted..." event which doesn't require player base). The event is randomly selected from available events.

Straight from the wiki... but that site is outdated. So some stuff is diffrent now.

Like the Ground is shaking, when the trolls come, it says ! troll and Elder needs to be killed, Talked with crew of Valheim, and they changed that. 1 troll kill "can" start the event now.