Complex sentence: They incited their younger brother, Gutthorm, to kill Sigurðr, by giving him a magic potion of enraged him, and he murdered Sigurðr in his sleep of Dying, Sigurðr threw his sword at Gutthorm, killing him.
Split into simpler sentences: They incited their younger brother, Gutthorm, to kill Sigurðr, by giving him a magic potion of enragement, and Gutthorm murdered Sigurðr in his sleep. As he was dying, Sigurðr threw his sword at Gutthorm, killing him.