The Proto-Indo-European root *bʰeh₂- "to speak" became Proto-Germanic *ƀannan "proclaim" (as in English ban), and this was thought (according to the AHD) to have been borrowed into Old French as ban "proclamation, publication, summons". This became Old French banal. According to the OED, banal in English first meant "Of or belonging to compulsory feudal service", then "Open to the use of all the community", which then came to mean "Commonplace, common, trite; trivial, petty".
*bʰeh₂- became Latin fārī "to speak", the past participle of which is fātum, literally meaning "that which has been spoken". It was used to mean "prophecy", similar to Greek προφήτης (prophētēs) "prophet", from φημί "to speak", also from *bʰeh₂-.
The Latin plural of fātum, fāta, was used in the singular to mean "fairy, Fate", and this became Old French faerie and English fairy.