Applied Thunderlord III to a sword and it triggers on every hit in the overworld with no roof. (it ends up killing the user as well).

I haven't been able to tie it to a specific scenario. Thunderlord 3 shouldn't even be a 100% chance (I see 30% in the source)

I am unable to replicate this. Can you provide more info, like is this a server or singleplayer? Do you have any other enchantments on the sword? What version are you on?