Tafsir of Qaf 50:23

{And his companion will say} He is the devil who was assigned to him, as in the verse: "We assign to him a devil, so he is a companion to him" (Az-Zukhruf: 36). This is confirmed by the Almighty’s saying: "His companion will say: 'Our Lord, I did not make him transgress'" (Qaf: 27).

{This is what is with me, prepared} Meaning: This is something I have with me, in my possession, prepared for Hell. The meaning is that one angel drives him, another testifies against him, and a devil—who is his companion—says: "I have prepared him for Hell and readied him for it through my seduction and misguidance."

If you ask: How is this speech parsed? I say: If you consider “ma” (what) as an adjective, then “‘ateed” (prepared) is its descriptor. If you consider it a relative pronoun, then it is a substitute (badal), a second predicate, or the predicate of an omitted subject.

{Cast into Hell every obstinate disbeliever, hinderer of good, transgressor, doubter, who set up another god with Allah; so cast him into the severe punishment.}