This is an idiom. "The heart of" means "the middle of" or "the innermost part of," and "heaven" here refers to the sky. AT: "with a fire that went up to the sky" (See: [[rc://en/ta/man/translate/figs-idiom]])
# with darkness, cloud, and thick darkness
Here "thick darkness" describes the cloud. AT: "with a thick, dark cloud" (See: [[rc://en/ta/man/translate/figs-hendiadys]])