en_tn/jer/48/15.md

Moab will be devastated and its cities attacked

This can be stated in active form. Alternate translation: "The enemy army will devastate Moab and attack its cities" (See: rc://en/ta/man/translate/figs-activepassive)

its finest young men have gone down to the place of slaughter

The best men of Moab going to battle and being killed is spoken of as if they were animals taken to a place to be slaughtered. Alternate translation: "their fine young men will all be slaughtered" or "the enemy army will kill all of the finest men of Moab" (See: rc://en/ta/man/translate/figs-metaphor)

This is the king's declaration

Here "king" refers to Yahweh. This can be stated in first person. See how you translated this in Jeremiah 46:18. Alternate translation: "This is what I, the king, declare" (See: rc://en/ta/man/translate/figs-123person)