Here "threw it down to Sheol" is an idiom that means to kill. "The pit" refers to the grave; because the grave was thought to be the entrance to the world of the dead, the pit also represents that world. See how you translated this in [Ezekiel 31:14](../31/14.md). Alternate translation: "when I killed the cedar and it went to Sheol with everyone else that is dead" (See: [[rc://en/ta/man/translate/figs-idiom]] and [[rc://en/ta/man/translate/figs-metonymy]])
"that were already deep down in the ground." This is an idiom. Alternate translation: "that had already died and are now in Sheol" (See: [[rc://en/ta/man/translate/figs-idiom]])
"the best trees of Lebanon that everyone would want; trees that got a lot of water." This is describing the trees of Eden that were in the lowest parts of the earth.