en_tn/2ch/21/08.md

In Jehoram's days

"While Jehoram was king"

from the control of Judah

"against Judah's control"

set a king over themselves

The word "set" is a metonym for choosing, and the words "over themselves" is a metonym for "to rule them." AT: "chose a king to rule over them" (See: rc://en/ta/man/translate/figs-rpronouns)

crossed over

"crossed the border into Edom" or "went to Edom"

It was night when he rose up ... surrounded him and his chariot commanders

It may be helpful to reorder this sentence and to add implicit information. AT: "The Edomites surrounded him and his chariot commanders, but Jehoram and his army fought against them and escaped during the night" (See: rc://en/ta/man/translate/figs-explicit)

to this present day

This refers to the time when the book of 2 Chronicles was written. See how you translated this in 2 Chronicles 5:9.

Libnah

This was a town in Judah. (See: rc://en/ta/man/translate/translate-names)