An interesting one! From a look through its entry in the Geiriadur Prifysgol Cymru, the authoratitive etymological dictionary of the language, an old meaning was indeed 'slaughter'. The most recent quotes using that meaning, excluding entries in some old dictionaries, are several hundred years old. It then seemed to be used to refer to the last act of a play or drama, as 'catastrophe' - a technical dramatical term. A similarly old meaning was 'a liitle bit, a small amount'.
But for a long while it has been used as mis Tachwedd. - the time of the last autumnal products in the agricultural cycle, perhaps?
Neither. dd sounds like the English voiced /th/ sound in 'the', 'those', 'these'.
The series of videos recommended in first post in the 'Pronunciation' discussion here are essential viewing, really. They are only about five minutes each and we recommend that you go through them all regularly during the course.