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)

Within the beautiful sea, the handsome King of Laedonti deserts his luxurious life under the sea; He rises to land in search of the princess who shines like the sun. In reverse of the little mermaid story we all know, comes the start of an intense love story. (Source: Esthetique)

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)