Hades, Realm Of The Dead
The underworld world is the underworld, Hades, or the realm of the dead.
This word is a bit poetic, and mostly used in writing, particularly in contexts like Buddhism, spirituality, and ancient mythologies. It's usually interchangeable with 冥土 (which is also in this level), but 冥土 is used more in certain set expressions, which we'll mention in the page for that word!
This is a jukugo word that uses the on'yomi readings of the kanji. You learned those with the kanji, so you should be able to read this on your own.
Only the souls of the dead can enter the underworld.
Hecate is the goddess of the underworld in Greek mythology.
I can hear a message from the god of the underworld coming from the realm of the dead.