Katachthónios (καταχθόνιος) means under the earth, subterranean — beings in the realm of the dead. Appears once in the NT in the great Christ-hymn.
Philippians 2:10: 'at the name of Jesus every knee should bow, in heaven and on earth and under the earth.' The cosmic scope of Christ's lordship encompasses not just the living but even the dead — every power, principality, and being must bow. The one who descended lowest (the cross, death itself) has been exalted highest. No realm is outside Christ's sovereignty — not even death itself. Revelation 5:13 echoes: 'every creature in heaven and on earth and under the earth' praises the Lamb.