Katotero (G2736) is a comparative adverb/adjective meaning 'lower' or 'into the lower parts.' In Ephesians 4:9, Paul uses this to describe Christ's descent: 'He also descended into the lower parts of the earth.' The word sits at the center of a grand Christological movement: descent then ascent.
Ephesians 4:8-10 quotes Psalm 68:18 and builds a theological argument about Christ's humiliation and exaltation. The One who ascended on high 'also descended into the lower parts of the earth.' Whether this refers to the Incarnation itself (descending to earth, which is 'lower' than heaven) or to Christ's descent to Hades between death and resurrection has been debated by theologians since Origen. Either way, the point is unmistakable: the ascended Lord is precisely the one who descended lowest. No depth of human experience — not death, not the grave, not the uttermost shame — was outside His reach. He descended so that He might fill all things (Eph 4:10).