al-Ṭūr

وَإِنَّ لِلَّذِينَ ظَلَمُوا عَذَابًا دُونَ ذَلِكَ وَلَكِنَّ أكْثَرَهُمْ لَآ يَعْلَمُونَ

47. And indeed, for those who do wrong, there is another chastisement besides this in this world or in purgatory; but most of them know not.

At that time, disbelievers shall deserve torment for two reasons. Firstly, they deserve torment for their disbelief thereby they associated other objects of worship besides Allah, since disbelief leads to torment. Secondly, they not only have wronged themselves through depriving themselves of virtues and felicity but also have they left no stone unturned to wrong the Noble Prophet (S) and believers. Thus, their torment shall be different from that assigned for all disbelievers.

"Another chastisement besides this"

may indicate this worldly torment including being killed or captivated or it may designate torments descended from the heaven, like those sent down upon the peoples denying their Prophets (as) aforetime, including famine, awe-inspiring cries heard from the heaven as the consequences of their disbelief and harming Messengers of Allah. Such torments are besides those in store for them in the Hereafter. Such disobedient peoples tasted Divine Wrath in this world and it led to their perdition.

It may be noted in passing that according to Tafsir Qumi,

"another chastisement besides this"

makes a reference to the torment to befall those who wronged the family of the Noble Prophet (S) upon return.