As I already commented on a previous post, it is acknowledged that the Mabinogion is the most valuable written source where the character of Rhiannon appears.
Rhiannon features in both the first and third branches of the Mabinogi, the tales of:
(a tale that revolves around his friendship with the ruler of the nether-realm, his finding and gaining Rhiannon as a wife, the birth, loss and re-discovery of his son) &
(a tale that recounts how Pryderi ,-who survived the war in Ireland-, offers his own realm to Manawyddan and gifts him his mother, Rhiannon to be Manawyddan's wife).
"Rhiannon: Goddess Of The Dead"
According to J A MacCullogh, in his book "The Religion of the Ancient Celts" the story of Rhiannon is even more complicated, since the Mabinogion was not originally written down, but was memorised and retold countless times, before it was first translated by Lady Charlotte Guest of Dowlais, Merthyr Tydfil, in the 19th century, and moreover it is well accepted that ancient Celts never wrote down their history or mythology, so much of what we think we know is told through oral recountings.
Though there is little that is mythological here, it is evident that Pwyll as a mortal assumes the qualities of a God by marrying Goddess Rhiannon, whose early importance, like that of other Celtic Goddesses, appears from her name, a corruption of Rigantona, "Great Queen."
Rhiannon was originally the daughter of Hefydd Hen (aka Heveidd Henn , Hefaidd Hen), "the Ancient", himself a Welsh God or hero though no recorded story explains his mythical function, and was probably a very old Goddess.
As shown on Pwyll Pendefig Dyfed Pryderi was her son, and two candidates are given for the role of his father: "Pwyll" and "Manawyddan".
MacCullogh suggests that Manawyddan and Rhiannon were aspects of older gods and parents of Pryderi, the dying and reborn god. (A theme which occurs in many religions.) Manawyddan becomes Pryderi's foster father, in some versions of the story, and later, Pryderi gives Rhiannon to Manawyddan in marriage.
As in Irish myth, we discover here the theme of a mortal helping a deity in the Other World while assisting Arawn, king of the Annwn (the Welsh Otherworld or Underworld), and while doing so meets Rhiannon who also retains the Goddess Epona on her character, and appearing unto Pwyll riding a white mare from a magic hillock. The recurrent motif of the "Fairy Bride".
Thus it is considered that her divine qualities as Rigantona “The Great Queen” were diminished in order to marry a mortal man. Her rise and fall and change into death Goddess in either case, it illustrates the dangers that befall Goddesses when they take up with mortal men.
Being connected to Mórrígan the Goddess of War whose name resembles hers (Mórrígan has some of its linguistic roots) especially because Rhiannon is associated with birds, in her case three magical birds who fly always around her shoulders, singing so sweetly that the dead awaken and living fall into trance.
"The Religion of the Ancient Celts" by J. A. MacCulloch ("The encyclopedia of Celtic mythology and folklore" by Patricia Monaghan (2004)