Masca

Although this was released after the 12 volumes of Masca, it is a prequel to the main story. It shows how Caien became a demon lord, about his relationship with Rakin, The King of Speira, and the love triangle between the 2 with the beautiful Rea of the mascan race. Asarella also apears, but the story seems to be concentrating more on Rakin and Rea. (Source: Evil Flowers)

Did you think Masca: The Tale of the Great Mage of Hessed was over? Masca: the Kings is the sequel of Masca, in which Asarella goes on a journey to save Kainen. However, the first two volumes focus quite a bit on the development between Rea and Rakin (Kainen's beautiful ex(?) and another demon king). (baka-manga updates)