Jamieson, Fausset and Brown on Isaiah 57:9

9. the king--the idol which they came to worship, perfumed with oil, like harlots (Jr 4:30; Eze 23:16, 40). So "king" means idol (Am 5:26; Zep 1:5); (malcham meaning "king") [Rosenmuller]. Rather, the king of Assyria or Egypt, and other foreign princes, on whom Israel relied, instead of on God; the "ointment" will thus refer to the presents (Ho 12:1), and perhaps the compliances with foreigners' idolatries, whereby Israel sought to gain their favor [Lowth] (Is 30:6; Eze 16:33; 23:16; Ho 7:11).

send ... messengers far off--not merely to neighboring nations, but to those "far off," in search of new idols, or else alliances.

even unto hell--the lowest possible degradation.