From testing I've done, the only thing that affects base attack event progression is killing the bosses IN THAT WORLD. If you live in a world in which you haven't killed any bosses, but you've brought the boss trophies over from some other world to mount on the shrine, you can have access to all the boss powers, but will only ever get "Eikthyr rallies the creatures of the forest."

Haven't been bothered to go to a new world just to kill Moder or Yagluth, but that would be an interesting test.