What is "runelight"

Runelight is a 2011 fantasy novel by Joanne Harris and is the second in her RUNE series, following her previous novel, Runemarks. The book centers on Maddy Smith, and her twin sister, Maggie Rede, both on opposing sides of a war between the new gods and the old, who must both face powerful forces in order to save everything they hold dear. Harris began working on the sequel due to feeling "that it wasn't finished as a story, that there was one chapter in the story that might continue".