from doing reverse boss order people found out that raids dont happen until you kill the first boss.

from the FAQ of this mod
https://www.nexusmods.com/valheim/mods/278
FAQ:
Can I just have no raids?
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Yes. Raid activation happens serverside, making this even simpler, you should only need this mod serverside if you just want no raids. (You can also use Event Enhancer for this)

Set configuration "EventTriggerChance=0" or

Set configuration "RemoveAllExistingRaids=true", as long you don't have something custom, this will ensure no random raids.