The term "agent of downfall" refers to someone or something that causes destruction, harm, and failure. There are several synonyms that can be used instead of this phrase, depending on the context and tone of the sentence. For instance, the word "destroyer" can be used to refer to someone who causes ruin or annihilation. Another synonym for "agent of downfall" is "harbinger," which means a person or thing that signals the beginning of a destructive event. "Troublemaker" and "saboteur" are also excellent replacement words that describe someone who deliberately causes problems or undermines success. Other options include terms like "bane," "wrecking ball," and "undermining force".