Complex sentence: As he enters, Jon is attacked by one of the dead rangers he found, now resurrected as a wight and despite cutting one of its arms and impaling it with his sword, it refuses to fall.
Split into simpler sentences: As he enters, Jon is attacked by one of the dead rangers he found, now resurrected as a wight. The undead creature proves immune to Jon's sword; even after losing one of its arms and being run through, it refuses to fall.