With OBD standard (by the moment) I calc'd Large Planet+ level and FTL for Primarch tier. https://onelastforum.com/threads/warhammer-40k-calcs.4921/ There are a couple of solar system level feats for Daemons in the Warp but that generate contradictions so it is controversial. His diseases have conceptual properties and affect even inorganic matter, so he should ignore durability to some extent with them. The weakness is that they take a few seconds to make effect and as we saw with Jaghatai, defeated Mortarion before his diseases kill or corrupt him.