Gert wrote:It is believed Magnus was originally the intended Throne sitter but the Emperor sat in it before the Webway project blew up. The Emperor's last command to Dorn was to place Him on the Throne, which Dorn did. Whether or not there was some grand plan with regards regeneration or whatnot is anyone's guess.
Iirc, to add onto this, not only was the intention for Magnus to do that, but he and his legion was going to basically be the "webway police" with the tsons specifically being the ones to help guard and monitor said web way.