Complex sentence: As a demonstration of what kills the wights, Jon lights the wight's hand on fire and stabs it in the heart with dragon glass, and asks for Cersei's help in the "great war" against the White Walkers.
Split into simpler sentences: As a demonstration of how to kill the wights, Jon lights its hand on fire and stabs it in the heart with dragon glass. Jon asks for Cersei's help in the "Great War" against the White Walkers.