Simplify the following text for a Grade 10 native English-speaking person.
Original: Pope Gregory VIII called for a crusade , which was led by several of Europe 's most important leaders : Philip II of France , Richard I of England ( aka Richard the Lionheart ) , and Frederick I , Holy Roman Emperor .
Simplified: Pope Gregory VIII started a new crusade , led by several of Europe 's kings : Philip II of France , Richard I of England and Frederick I , Holy Roman Emperor .