Island of the Mighty retells the last branch of the Mabinogion, the story of Gwydion, Arianrhod, Llew Llaw Gyffes, Blodeuwedd and Goronwy. It begins with a retelling of stealing the pigs belonging to Lord Pryderi. Gwydion uses this to provoke war, allowing his younger brother to rape the king's footholder. This also leads to the death of Pryderi, which doesn't endear Gwydion to the reader who has also read the retellings of the other three branches — and also to the disgracing of Arianrhod and the birth of Llew Llaw Gyffes.
The themes Evangeline Walton explored in the other books come to fruition here, as power passes more and more from women to men, even power over birth and the rearing of children. Arianrhod is not very sympathetically dealt with, I have to say: often Walton's work suggests that the passing of women's power is a bad thing, but Arianrhod is capricious and unkind, considered by characters and text unnatural — for the crimeof not having wanted to bear a child! Blodeuwedd isn't treated with much sympathy here, and the other women are barely characters.
It's hard to sympathise with most of the characters here, particularly as they stir up war, steal, lie and trick each other. I still enjoyed it as a retelling and think Walton dealt well with the material, but I wish she'd been kinder to Arianrhod and Blodeuwedd, who were both unable to fit in the patriarchal society that wanted power over women's bodies, and expected them to abide by two conflicting sets of rules.