Wolfick
Styx was surprised but he was probably already expecting something from the human army to restrain Aura. For these types of spells, their usual typical critical weakness is that they had to keep channeling if it's a continuous channeling spell and if it isn't it usually requires a large amount of sacrifice and it couldn't be used multiple times. What I am interested in now is if the human army actually know that there's actually a second 5th tier protecting this dungeon cause the messenger got intercepted and Styx didn't know about that thus he assumed that the kingdom already knew that there were two monster lords guarding the dungeon. Also I wonder whether it was the goddess or the demon god or some other unknown forces who intercepted that messenger bird.
Even if I use my backside to think, I will still be able to guess that the Kingdom must have some sort of trump card against the Monster Lord since they went to war with him despite knowing the Monster Lord's existence. For the 4000 years old guy with plenty of "Experience" as compared to my backside, can't think of that even after hearing about "Plan Eternity Lock"? The author sure made him dumb on purpose because plot. Bad writing.