There are several synonyms for the word "fatalness". Some of the most common synonyms include lethality, deadliness, fatality, mortalness, and lethiferousness. These words all convey the idea of something that can cause death or mortality. Other synonyms include harmfulness, dangerousness, perilousness, and noxiousness, which suggest that something is harmful or potentially deadly. Still others include finality, irrevocability, inescapability, and inevitability, which suggest that something is inevitable and cannot be avoided. Whatever word you choose to use, it's important to remember that fatalness is a serious matter and should always be treated with the utmost caution and respect.