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The term sultan, which primarily signifies "power" or "authority", has here - as in many other places in the Qur'an - evidently the meaning of "argument", synonymous with hujjah (Ibn 'Abbas, 'Ikrimah, Mujahid, Ad-Dahhak, all of them quoted by Tabari): in this case, an argument or arguments against the idea of life after death and, hence, of divine judgment.
The intensest agony is when the soul loses power over itself, when the personality tries to realise itself in new conditions and cannot: this is life in death.