While lost in one of those Wikipedia wormholes I so often find myself in a few years back, I encountered the Holly King and the Oak King. Reading further on the subject across various books and websites, I discovered that they are speculative motifs of Celtic (and possibly wider European) folklore representing the cycle of the year. The idea was first proposed by Sir James George Frazer in his book The Golden Bough and expanded upon by Robert Graves in The White Goddess. According to Graves, the Oak King represents the ‘light side’ of the year, gaining in strength from just after the winter solstice until he reaches the peak of his power at the summer solstice. After that, the tables turn, and slowly the Holly King grows in strength as the Oak King wanes. The two kings are engaged in a never-ending battle, alternately gaining the upper hand as the year progresses. This idea of an endless cycle of waxing and waning power is typical of what is known of pre-Christian Celtic beliefs, which focused heavily upon the cyclic nature of the seasons.
Graves identified several pairs of male folklore figures, including Sir Gawain and the Green Knight from the Arthurian legends, which he believed represented the Holly and Oak Kings. In homage to their endless battle, they are hidden within the pages of A Skin of a Dragon, subtly represented by their respective trees. Perhaps you can identify them….