The spelling of the phrase "haddest mercy on" can be explained using the International Phonetic Alphabet. In this phrase, the "h" is pronounced as /h/, the "a" as /æ/, the "d" as /d/, the "e" as /ɛ/, the double "d" as /d/, the "s" as /s/, and the "t" as /t/. The word "mercy" is pronounced as /ˈmɜːsi/ with the stress on the first syllable. The phrase "haddest mercy on" is an old-fashioned way of saying "had mercy on" and is commonly used in historical and literary texts.