Annwn was the paradisaical Otherworld in Welsh Mythology. Ruled by Arawn, or much later by Gwyn ap Nudd, Annwn was originally more like the Irish Tir na Nog; a home of spirits, deities, and fairies. Dangerous to humans, and not necessarily following the same laws of space or time as the human world, Annwn was the home of the Tylwyth Teg, the Fair Folk.
Later mythology would reinterpret Annwn as the Land of the Dead, possibly under the influence of Christianity.
In the Cad Goddeu, some of the inhabitants of Annwn are described:
“There passed me by a wide-mawed beast
bearing a hundred heads
one mighty host
’neath the root of its tongue
and another host dwells
in the napes of its neck
A black-groined toad
bearing a hundred claws
a mottled, ridged, serpent
with a hundred souls, by their sin
tortured in the folds of its flesh” (This last is believed to be the Dragon of Annwn.