Fenrir is not the weak transcendent that he made himself out to be. He was also not in any danger when the mountain collapsed on him. He was alright after all, and he could have dug himself out had Xigger not saved him.
If Ragnarok knew the true depths of the Emperor's power, then he wouldn't want to be anywhere near him. Then they wouldn't be able to speak and the Emperor won't be able to sell him on the idea of becoming a herald to the fountain of life.
It is not Ragnarok's fault that he was fooled. He is cautious enough. But he relies on his perception and his perception can be fooled. Those who are proficient with the law of life are especially good at concealing the aura and signs of life. Ragnarok didn't know he was digging into the body of an ice golem and he didn't know that the Emperor he was speaking with was a king of law.
