Evil gods work together to make monsters.
If an evil god makes one on their own, it will be very weak, and have a very pronounced Achilles’s heel.
The more status an evil god has, the more likely it is that other gods will work on a disciple of evil with them.
This is what Corax was thinking of when he worked with Mithras to make the cult; Mithras had decided to give Corax a chance, because Corax was very low status, and Mithras was significantly higher; if their work together was a success, Corax would finally get the respect he deserved!
Together, they made the cult. The cult was able to provide Mithras with blood sacrifices and allowed Corax to extinguish the light of castle Adiart.
The only problem was that Mithras was tiring of the cult; she received blood sacrifices, but had to use that energy to kill someone else bloodlessly.
Corax worried that Mithras would give up on the cult; if such a high status god rejected Corax, no other gods would work with Corax for millennia!
So he devised the acolyte.
The acolyte allowed the cult to give Mithras blood without making her do anything. This was just enough to bring Mithras’ attention back to the cult.
In this way, the acolyte was the must important cultist, as without him, the cult would only have the support of the weak god Corax.
Corax would sleep easy, knowing that Mithras would take it from their. Not that gods sleep…