Isaiah 57:9

Some Israelites had also traveled far from home to worship other gods. This may be a reference to making political alliances with other nations and then worshipping their gods with them (cf. Ezekiel 23). The king in view may be the most prominent foreign ruler at the time Isaiah wrote this prophecy. These political trips involved great distances. The negotiators would take the oils and perfumes used in the worship of foreign gods with them. Over time these instances of idolatry had increased. But instead of going to foreign nations, Isaiah said these envoys were really going to Sheol because God would slay His people for their unfaithfulness to Him.