Maniac and maniacal are ‘grammatical form heteronyms’ – meaning the main part of the word changes its pronunciation in different grammatical forms but keep the same meaning.
Maniac means: a person exhibiting extremely violent behavior. And has its roots in the early 16th-century word ‘mania’ which comes from the Greek ‘maniakos’ meaning ‘madness’ or ‘frenzy’. It is actually from the proto-Indo-European word ‘men’ meaning to think, which also gives us ‘mind’.
Maniacal is the adjective form of ‘maniac’ and essentially means ‘like a maniac’. The ‘al’ suffix is common when turning words into adjectives in the English language, it comes from the Latin ‘alis’ which was used in the same way, and of which we still see many examples today.