The correct spelling of the word is arguably "most deadly" rather than "most deathful." It is important to note that "deathful" is not a commonly used word in standard English. The correct pronunciation for "deadly" is /ˈdɛdli/, with stress on the first syllable. The word "most" is pronounced /moʊst/, with stress on the second syllable. Therefore, the correct pronunciation for "most deadly" is /moʊst ˈdɛdli/.