Not sure if this is really the right section, but it was sparked off by a quote in 'Wyrd Sisters'.

Who is Destiny? One of the Anthropomorphic Personifications or Gods, presumably, but what is his/her function? Apparently not Fate, because in WS, talking to King Verence Death refers to "FATE. DESTINY. ALL THE REST OF THEM" leaving him to pass on the news that the King is to become a ghost. I'm sure there's another quote whether Death refers to having an appointment with Destiny. So, any suggestions as to his/her identity and function?

I don't think it's ever been spelled out, it's just one of those things that Discworld would have a god/anthropomorphic personification of.

Destiny is a her it's mentioned several times and she does what you would normally expect Destiny to do.