Who is the woman in the nether-world and why is she there?

This question is based on Proverbs 5 from the JPS translation:

Proverbs 5:3-5 (JPS)
For the lips of a strange woman drop honey, and her mouth is smoother than oil;
But her end is bitter as wormwood, sharp as a two-edged sword.
Her feet go down to death; her steps take hold on the nether-world;

It is clearer in a more modern translation:

Proverbs 5:3-5 (NIV)
For the lips of an adulteress drip honey,
and her speech is smoother than oil;
but in the end she is bitter as gall,
sharp as a double-edged sword.
Her feet go down to death;
her steps lead straight to the grave.

So the “strange woman” is sinful and seductive. The verse is not referring to a specific woman, but to anyone who behaves in this manner. The word translated “nether-world” is Sheol in Hebrew, which simply means “covered place” or “grave”. Some Bible versions translate the word as hell.

The woman will go to the grave because that is where such behaviour eventually leads. The warning from Proverbs 5 is that we should avoid being like the adulteress or we will end in the grave as well.