“flesh that mirrors Him” -
This is an allusion to Genesis 1:27, which says “So God created man in his image, in the image of God created he him.”
Tantalus is a king from Greek mythology who angered the gods for stealing ambrosia and nectar and sharing it with his people, in some versions, or for offering his own son up as a sacrifice and trying to serve him up as food for a banquet the gods were attending. Tantalus (whose name gives us the English word “tantalize”) was punished by Zeus for these actions. He had to stand in a pool of water under a fruit tree. However, whenever he tried to eat the branches with the fruit would move out of reach. Whenever he tried to drink, the water receded.
Another figure from Greek mythology, King Sisyphus was punished by Zeus for his hubris or excessive pride. He was condemned to rolling an enchanted boulder up a steep hill. Every time he reached the top, the boulder would roll back down the hill. In this way, Sisyphus was condemned to a futile and repetitive fate.
“What awful brain compels His awful hand” -
This could be an allusion to William Blake’s poem “The Tyger,” which is also about the frightening things an all-powerful God chooses to do and create:
Tyger Tyger, burning bright,
In the forests of the night;
What immortal hand or eye,
Could frame thy fearful symmetry?