Tafsir of Ar-Rahman 55:43

The phrase "This is Hell..." is the content of a speech that is implicit, which is conjoined to His (the Exalted) saying: "They will be seized..." meaning: "And it will be said: 'This is Hell...'"

Alternatively, it is a new sentence (inaugural) in response to the question: "What will be said to them?" because it is a place for reproach and rebuke.

It may also be a circumstantial clause (hal) related to the possessors of the forelocks, based on the assumption that the implication is "their forelocks" or "the forelocks from among them." Anything intervening between these two possibilities is a parenthetical remark.

The original phrasing was intended to be: "Which you used to deny," but the shift to "Which the criminals deny" was employed to signify the continuity of that denial and to clarify the aspect of their rebuke and the reason for it.