In older English “-eth” performed the same function as S in the third person singular present of verbs, as in “my cup runneth over.” People jokingly trying to make speech sound antique often add “-eth” randomly to plurals, tenses, and person with which it never belonged. Unless you are trying to make your characters sound stupid, don’t have them say things like “my cookies crumbleth,” “the window broketh,” or “you charmeth me.”