Originally posted by Samoth:

it'sa hanging the trophies i thought. thats the trigger for flagging.

I thought the same Samoth, but no, one has to physically kill the bosses for the applicable invasion event to be triggered on the map. Tis nice having Eikthyr's & Bonemass' buffs without having the usual accompanying invasions. Let's me explore and build without hassle or having to initiate console commands. Really got lucky with this map seed too. It's got large swaths of plains, so I can ride lox from coast to coast, and it has lots of swamp, so I have lots of iron on hand for building and such.