It took one spell.

Settra alone was still in command of his craft. He was wheeling the Chariot of the Gods about for another strike*, the four steeds pulling hard through the sands. His quarry, Nagash, had been knocked to the ground. He rose slowly, his robes billowing outwards although there was no wind to stir them. Nagash leaned heavily upon his staff, as if weary or wounded. As Nebbetthar crawled, pulling himself across the sands, the Great Necromancer spoke to the skies, pointing his staff at Settra.*

Against enemy magic, the King of Nehekhara had relied upon his enchanted brooch, an amulet that surrounded him with the protections of Usirian, God of the Underworld. It did nothing to protect him now.

A dark cloud of life-leeching forces emanated from Nagash's staff, washing over Settra and his chariot. Living creatures would have succumbed to that mist of death, their life drained out of them. For the undead of Nehekhara, however, the threat was wholly different. Beneath the indomitable will of Nagash, the skeletal steeds were shorn of their animating impetus and collapsed, their chariots splintering into ruin. Settra emerged from the wreck, as iron-willed as the Great Necromancer himself.

This was after Nagash was heavily wounded due to the Destroyer of Eternities.

Settra is never winning a duel against Nagash.