May I begin with that this is not my work and I take no credit to this work. I have typed this up to aid people on this site. All rights go to the author Sarah Bartlett. This information is found in the book "The Mythology Bible".
Annwn (also known as Anfwn, Affan and Anghar ) wa the Otherworld or Underworld in the Celtic myths. A mirror image of the mortal world, it was,however, timeless and formless.
To begin with, there were two rulers of Annwn called Havgan and Arawn. All the inhabitants of Annwn were spirits, demons or fairies who were of no substance and were only apparent in form when they traveled into the mortal world to trick, confuse or harm the humans they encountered. Annwn later became Christianized and identified as the land of departed souls
in the mabbinogion , Arawn wanted to destroy his rival Havgan and become the sole ruler of the other world. He knew he wasn't strong enough to win, but Pwyll - King of Dyfed- was. One day while out hunting, Pwyll's pack of hounds chased off another pack that was killing a stag. The owner of the stag was Arawn, and as a recompense for this insult , he persuaded Pwyll to rule the Underworld for a year while Arawn ruled of Dyfed. During this time, Pwyll easily killed Havgan and a year later they returned to their own kingdoms. As sole ruler of Annwn, Arawn was from there on seen as a tame and gentle ruler. In a much later myth, Annwn was ruled by Gwyn Ap Nudd, who lead a supernatural pack of hounds and escorted the souls of the dead to the Otherworld.
The Book Of Tailesin an ancient Welsh manuscript, which contains a poem which King Arthur and his nights travel through Annwn in search of the Cauldron of Plenty the eternal source of the Otherworld's delight and immortality. It is possible that this is the precursor to the later Grail myths. The text dates back to the 10th century CE, but scholars believe that most Welsh myth was passed on orally, and thus this poem could be as far back as the 6th century CE.
Annwn is said to be accessible at the mouth of the Severn river near Lundy Island or from the top of Glastonbury Tor, near the town of Glastonbury in the south west of England
Please let me know what you think,
I plan to fill this forum will lots of information.
My brightest Blessings,