John Gill Commentary Psalms 18:9

"He bowed the heavens also, and came down; And thick darkness was under his feet." — Psalms 18:9 (ASV)

He bowed the heavens also, and came down
To execute wrath and vengeance on wicked men; which is always the sense of these phrases when they go together; see (Psalms 144:6–8) (Isaiah 64:1Isaiah 64:2) ; The Targum is, "he bowed the heavens, and his glory has appeared"; that is, the glory of his power, and of his mighty hand of vengeance; for not his grace and mercy, but his indignation and wrath, showed themselves; for it follows,

and darkness [was] under his feet ;
the Targum is, "a dark cloud", expressive of the awfulness of the dispensation to wicked men; who are not allowed to see the face of God, are debarred his presence, and denied, communion with him, and to whom everything appears awful and terrible, (Psalms 97:2–5) .