Isa 57:9

57:9 the king. The name of their god, “Molech,” is similar to the Hebrew word for “king” and is probably what is meant here.

sent down. Isaiah describes the energy they spent in willing service of their idols.

Sheol. This charge of visiting the realm of the dead (5:14 note) is either a figure of speech to express the depths to which they sank, or a reference to necromancy (communicating with the dead).