The backstory:

Nerina's other great love finally died from the injuries during the battle in Nadrak between the Malloreans and the Nadraks. His relative inherits and plans to marry her off to some friend without her consent - somehing it indicates he has every legal right to do, even if hes an ass f9rndoing it. Mandorallen finds out and basically is about to start a war that will consume much of Arendia.

Garion, technically Belgarion, Overlord of the West is not having it. He shows up, throws lightning around to get attention, challenges all the others to a jousting duel. He runs through, some quite literally, most of the 'champions, chews everyone out and forces Nerina and Mandorallen to marry, after overcoming their objections.

After he gets back, Belgarath returns to Riva from wherever to chew out Garion for the lightning/thunderstorm/weather tampering