The Invocation of Gwyn ap Nudd

According to a 14th century Latin manuscript against divination, Welsh "soothsayers" would repeat the following:

ad regem Eumenidium et reginam eius: Gwynn ap Nwdd qui es ultra in silvis pro amore concubine tue permitte nos venire domum"

Translation (from Medieval Folklore):
"to the king of Spirits, and to his queen--
Gwyn ap Nudd,
you who are yonder in the forest,
for love of your mate,
permit us to enter your dwelling."

NOTES
"Eumenidium" is a term for the Furies, and a fair classical approximation of Tylwyth Teg, "The Good People", a Welsh euphemism for the fairies. Gwyn ap Nudd is traditionally the king of the Tylwyth Teg.

SOURCE:
Medieval folklore: an encyclopedia of myths, legends, tales, beliefs, and customs ed. Carl Lindahl, John McNamara, John Lindow. Oxford University Press, 2002. p. 190.