In the shadow kingdom dungeon, after advancing his monarch core to D rank, Evan wasn't able to increase its rank at that time.
When he tried to increase the rank of his monarch core at that time, it did not absorb the energy of the core because if its rank had increased at that time, his prime core which was just F rank would have shattered because of its pressure.
Currently, the situation was the same at that time.
Because of forming a connection with the shadow realm, and receiving its blessing, Evan's monarch core level was increasing at a rapid speed.
Because its level was increasing, his D+ rank prime core was not able to handle the pressure released by it.
It was the first time in history something like this was happening.
Anything like this never happened before, because of one simple reason.
No one among forbidden physique holders ever had a second core other than their monarch core!