Essence
Spirit, True Meaning
noun, の adjective
The marrow of a god contains their essence and spirit. I don't know how you found that out, it was probably really dangerous. You shouldn't mess with gods.
しんずい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
プロの神髄を見せつけられました。
He was able to show me the true meaning of being a professional.