Strong’s Greek
G2709 — katachthonios
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
καταχθόνιος
katachthonios — pronounced kat-akh-thon'-ee-os
from 2596 and chthon (the ground); subterranean, i.e. infernal (belonging to the world of departed spirits):--under the earth. see GREEK for 2596
1 occurrence in the KJV.
Philippians 2:10
That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth;