Dagdan is the King of Hybern's nephew and the twin brother of Brannagh. He travels alongside his sister to Prythian and resides in the Spring Court whilst examining the territory in A Court of Wings and Ruin.
Dagdan and Brannagh arrive in the Spring Court, alongside Jurian, in the beginning of A Court of Wings and Ruin. They are welcomed by Tamlin and live in his mansion for the duration of their stay. The twins are extremely close, and Feyre observes that they have a sexual relationship with each other. The twins attempt to penetrate the minds of Tamlin, Lucien and Feyre whilst dining in the mansion, but Feyre uses her power to shield herself and the others.
Whilst examining The Wall alongside Feyre, Lucien and Jurian, they happen upon three Children of the Blessed. Dagdan and Brannagh move to ravage the humans, but Feyre shields the mortals from the twins' power and the humans are able to flee. However, later on in the night, the twins hunt down the mortals and kill them anyway.
Later on in the book, when Feyre is making her way back to the Night Court and halts to save Lucien from Ianthe, the twins appear and attempt to take Feyre as their hostage back to Hybern. Feyre is weakened by faebane at the time, poisoned by the twins themselves earlier on, but with Lucien's help, she manages to defeat, and kill, both commanders.