Complex sentence: Derfel met Lancelot and Galahad, and found Merlin in Ynys Trebes, and when the city was taken by the Franks Galahad returns to Dumnonia with Derfel and Merlin on Merlin's boat.
Split into simpler sentences: Derfel met Lancelot and Galahad, and found Merlin in Ynys Trebes. When the city was taken by the Franks, Galahad returns to Dumnonia with Derfel and Merlin on Merlin's boat.