There are many words and phrases that can be used to describe the feeling of being "overcome with horror". Some possible synonyms include "terrified", "horrified", "aghast", "horrifically shocked", and "petrified". Other related terms might include "appalled", "dismayed", "aghast", and "aghast". Ultimately, the choice of word will depend on the specific context and the tone and style of the writing. Some writers might prefer more understated and nuanced words, while others might opt for more dramatic and intense language to convey the full force of the horror and fear being experienced.