Guardians of the West by David Eddings

Garion has now fulfilled his task (or so he thinks) and has slain Torak. Garion and Ce'Nedra are now king and queen of Riva and the world seems to be in order again. But they don't get much time for relief. Something new and unknown is afoot and Garion and Ce'Nedra's son is kidnapped. A new adventure lies in front of them...