I think the Starpriest's ability can let you double the mortal wounds inflicted this way by Kroxigors.
If you read the AoS rules, under the description of Mortal Wounds (4th page) it says
Some attacks inflict mortal wounds.
Do not make hit, wound or save rolls for a mortal wound – just allocate the wounds to models from the target unit as described above.
So let's see what this "as descibed above means".
From Making Attacks - 4. Determine Damage:
Once all of the attacks made by a unit have been carried out, each successful attack inflicts a number of wounds equal to the Damage characteristic of the weapon. Most weapons have a Damage characteristic of 1, but some can inflict 2 or more [...]!
It means a Mortal Wound IS NOT a single uncancellable damage, but an uncancellable wound which deals as many damages as indicated by its weapon's stats.
So, in our case this Kroxigors' ability would deal a total damage equal to
[(Your die roll) - (Your opponent's die roll)] x (Vice-like Jaws' damage characteristic).
If no Starpriest has used his "Serpent Staff" ability that last parenthesis is equal to 1.
On the contrary, if this ability is active, you have to double the damage characteristic, so put 2 instead of 1.
Example 1: "Serpent staff" ability NOT activated, "Jaws like a Steel Trap" activated by a 6+ to Wound roll.
Then you roll a 5 and your opponent a 3.
The total amount of uncancellable Damage inflicted to the enemy model is [(5) - (3)] x (1) = 2.
Example 2: "Serpent staff" ability activated, "Jaws like a Steel Trap" activated by a 6+ to Wound roll.
Then you roll a 5 and your opponent a 3.
The total amount of Damage inflicted to the enemy model is [(5) - (3)] x (1 x 2) = 4.
That's my opinion.
