"the key that unlocks the shaft of the bottomless pit"
# the shaft of the bottomless pit
Possible meanings are 1) "shaft" is another way of referring to the pit and describes it as long and narrow, or 2) "shaft" refers to the opening of the pit.
# the bottomless pit
This is an extremely deep narrow hole. Possible meanings are 1) the pit has no bottom; it continues to go down further forever or 2) the pit is so deep that it is as if it had no bottom.
A huge furnace gives off a great amount of thick, dark smoke. AT: "like the great amount of smoke that comes from a huge furnace" (See: [[rc://en/ta/man/translate/figs-simile]])
"Turned dark" here is an idiom for "changed to dark." AT: "The sun and the air changed from light to dark" (See: [[rc://en/ta/man/translate/figs-idiom]]).