Aha! I did replay the turn ten times, trying to do things differently.
My pretender is a Lord of the gates
(Here he is in all his glory)
Turn 1 : I Stygian Path to ermor capital, he dies in the void.
Turn 2 : I stygian path to my capital, he dies in the void.
turn 3 : I stygian path to one of my provinces (with positive dominion) with no castle, he dies in the void.
turn 4 - 8 : I try different approaches with shutting down dominions to check if there is a random generator problem. (he dies in the void)
turn 9 : I stygian path to my capitol WITHOUT any troops. He survives and arrive unharmed in capital.
Turn 10 : I stygian path with removing a quarter of the undead troops and he dies in the void.
What theese few unscientific tests makes me belive is that if the army is big enough, the caster will be slayed regardless of other factors when travelling with stygian path.
Now this is only a theory, could also have something to do with the horrormark, or that the army is mainly undead and the caster is not, and hence he get all the damage the army should have gotten from the poison or whatnot.
What I am convinced of though is that the spell isn't supposed to work as it does in my current game (which I saved for anyone to look into
here )