﴿ فَضْلًۭا مِّن رَّبِّكَ ۚ ذَٰلِكَ هُوَ ٱلْفَوْزُ ٱلْعَظِيمُ ﴾

“an act of thy Sustainer’s favour: and that, that will be the triumph supreme!”

a bounty from your Lord fadlan is a verbal noun with the sense of tafaddulan and is in the accusative dependent status because of an implied preceding tafaddala. That is the supreme triumph.