The Chestnut King (also known as the Faerie King) is the title given to the king of the faeren. He lives in the Second World in Glaston's Barrow, where he rules the faeren with a great degree of secrecy. In fact, most of the faeren living the First World believe the Chestnut King is just myth and that their true monarch is the Faerie Queen.
Although the Chestnut King rules the Faeren and lives with the Faeren, he cannot be a faerie himself. He is allowed to rule as long as he wishes and cannot die of age while he is king. At any time, he may crown his heir and step down as king, but then he will lose his immortality and must return to the First World; depending on how old he is, he might die shortly after returning.
Clovis was the name of the first non-faerie Chestnut King. He was a green man who came to power at a time when the faeren had such a hatred for the humans that Nudd described them as "curses with legs and arms and spite." Clovis named Nudd, another green man, as his successor and made Nudd swear an oath to him to be the guardian of the Second World until he met a worthy successor who was not a faerie.
After over three hundred years of ruling the faeren, Nudd provided Henry with military support, transportation, and the Blackstar for an attempt Henry was making to destroy Nimiane and rescue his friends and family from her. He did this in return for Henry's promise to return to the Second World after the affair was over and be immediately crowned as heir to the throne.
Henry, however, convinced Nudd to crown Fat Frank instead.