"Horror of horrors" is a phrase commonly used to express extreme shock or disbelief. While this phrase is quite effective in conveying the sense of terror or dismay one may feel, there are other words and phrases that could be used as synonyms. Some of the phrases that could be used in place of "horror of horrors" include "beyond belief," "unimaginable," "stunning," and "mind-boggling". Alternatively, words like "nightmare," "dreadful," "terrifying," and "ghastly" could serve as powerful synonyms in this context. Whatever the phrase or word used, its effectiveness will depend upon the degree of shock or disbelief necessitated by the situation at hand.