{خَالِدِينَ فِيهَا}
This is a circumstantial qualifier (*ḥāl*) related to the pronoun in {عَلَيْهِمْ} (upon them). The governing element (*al-ʿāmil*) for it is the implied concept of settlement (*al-istiqrār*). The pronoun [in "fīhā"] refers to the curse, or the punishment, or the Fire—even if it was not explicitly mentioned, due to the sufficiency of the curse as an indication of it.
{لَا يُخَفَّفُ عَنْهُمُ الْعَذَابُ وَلَا هُمْ يُنْظَرُونَ}
Meaning: They are not granted respite, and the punishment is not delayed for them from one time to another. Alternatively, it means they are not looked upon [with mercy], and no account is taken of them. The sentence is either an incipit (*mustʾnafa*) or in the position of a circumstantial qualifier (*ḥāl*).
