Complex sentence: Throughout the first season, Nick struggles to maintain balance with his normal life and his Grimm life, both tending to cross when he works cases that involve "Wesen", the creatures of the Grimm world.
Split into simpler sentences: Throughout the first season, Nick struggles to maintain balance with his normal life and his life as a Grimm. The two tend to cross when he works cases that involve "Wesen", which are the creatures of the Grimm world.