1 boss an undead Jotun possibly
1 boss is powered by a jotun skull? Which is some form of undead being

Big bugs maybe decimated these beings possibly pushing them out where one survived being king of fulings and some others died but bringing birth to swamps and biomass.

It's interesting to think if this giant race dieing is actually the reason odin has sent some vikings out into valheim. Maybe the Jotun were the previous protectors of valheim and now they're gone the enemies of Odin have gained strength again.

So were the bugs an implemented plan by some other third party (maybe Ashland or deep north boss) to get rid of the Jotun or was it just unlucky some bugs evolved to kill the jotuns. Mugin says they used to rule the tenth world and then were ousted by some other power.