After Melisandre ignites the Dothrakis' swords with fire, they begin the charge towards the White Walkers.
The Dothraki charge the enemy, but are swiftly annihilated; a few survivors, including Jorah, are able to retreat and regroup with the remaining armies. As they begin formation, a snowstorm quickly overwhelms the forces of the living. Eddison Tollett is killed saving Samwell Tarly. Deviating from their plan to intercept the Night King, the dragonriders arrive and burn the advancing wights as the living begin retreating into the castle but the Night King creates a winter storm just as Jon Snow is about to burn the White Walker generals, rendering the dragons ineffective as there is no visibility.
Part I: https://www.youtube.com/watch?v=xKjiOKTY_5E
Part II: https://www.youtube.com/watch?v=XoeN8z2gEkI