Annwn (Avalon, Annwfn) The Ancient Brythonic Celtic Otherworld - Celtic Mythology

The ancient Celtic world of spirits. What is it? Who are the Gods or Kings of the underworld? Is there a link between the Brythonic Kings Arawn, Gwyn Ap Nudd, Hafgan and Gwythyr fab Greidawl? Are they the true Lords of Darkness and of Light (The Holy King and The Oak King)?