Complex sentence: Maskull, Sullenbode and Corpang proceed in a direction to which someone looking for Muspel once went, but Corpang goes eagerly ahead whereas Maskull stops when he has a vision of Muspel-light.
Split into simpler sentences: Maskull, Sullenbode and Corpang proceed in the direction someone looking for Muspel once went. As they near their destination, Corpang goes eagerly ahead but Maskull stops, caught in a vision of Muspel-light.