Complex sentence: Thor's daughter, Þrúðr, was promised to Alvíss, however, Thor did not want Alviss married to his daughter, so he devised a plan to stop Alvíss from doing so.
Split into simpler sentences: Thor's daughter, Þrúðr, was promised to Alvíss. However, Thor did not want Alviss married to his daughter, so he devised a plan to stop Alvíss from doing so.