Complex sentence: In March 2005, Frisk was forced to early retirement from refereeing amid pressure due to death threats made against him and his family by Chelsea fans to retaliate his handling of Champions League match FC Barcelona vs Chelsea.
Split into simpler sentences: In March 2005, Frisk was forced to early retirement from refereeing amid pressure due to death threats made against him and his family. It is alleged that those death threats were made by Chelsea fans to retaliate his handling of Champions League match FC Barcelona vs Chelsea.