فَيُعَذِّبُهُ ٱللَّهُ ٱلْعَذَابَ ٱلْأَكْبَرَ
Fa yu'azzibuhul laahul 'azaabal akbar
God will punish them with the greatest punishment.
The comparative carries the weight here: a greater punishment implies a nearer one, and the phrase occurs only once more in the Qur'an, at 32:21, where the lesser is sent ahead so that people might still turn back. It follows immediately on the messenger being told he holds no power over anyone, which moves the whole matter of consequence out of human hands.