은 is a topic marker, indicating what the sentence is about or the topic of discussion. It can emphasize an element, contrast it with another, or set the context for what follows, highlighting the theme rather than the actor and conveying a sense of ‘as for…’ or ‘regarding…’.
그는 실망하여 태양이 되는 것이 진정한 힘이라고 생각하며 태양이 되기를 소원했습니다.1 Disappointed, he wished to become the sun, believing it would give him true strength.