a special char with a special question

i am currently working on a special char with will end up being what we have described as a demi primarch for my custom space marine chapter and i wont get into all there lure here but i do i 1 question

every one of the primarchs has a opposite main eg being dorn and petarabo (defence vs attack) yet there in not any that would work for magnus so i am thinking of making my char a black/null (having the pariah gene)

So how could u put this into a special rule to make it work how its ment to: Only two come to mind at the moment so any imput would be very helpful

1, it makes psyker powers harder to cast : so a warp charge 1 would become 2 etc

2,its makes it harder to harness so that 4+ becomes 5+

thx for reading and once again any help welcome

LOKI