They is not identified, but it can be “his enemies” or “his conquerors.”

The last clause of the verse (and he shall never see this land again) essentially repeats the last clause of verse 11 (“He shall return here no more”). Apparently most versions retain the repetition, though it is possible to drop it by placing verses 11-12 together, as Contemporary English Version does:

• Jehoahaz became king of Judah after his father King Josiah died. But Jehoahaz was taken as a prisoner to a foreign country. Now I, the LORD, promise that he will die there without ever seeing his own land again.

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on Jeremiah. (UBS Helps for Translators). New York: UBS, 2003. For this and other handbooks for translators see here .