From Evil Flowers:
The story shows how Caien became a demon lord, is 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.

Note: Read this after you read 'Masca' as this series jumps right into it with no introduction and you will be confused if you read 'Masca:The Beginning' first

A woman awaits the return of the man she loves from a war in ancient Greece.

From Udon Entertainment:

Remember the tale of the little mermaid that fell in love with the dashing human prince? Now, imagine the roles reversed, as a romance blooms between the beautiful Evyione and a dashing young merman. It's a unique twist on a timeless tale that is sure to leave you spellbound.

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).

From CPM:
The Devil's Castle was a dark and somber place, until the good-hearted apprentice ASARELLA stumbled on it in a goodwill mission, against her master's wishes--the archmage ELIWHO. Things sour quickly, as Asarella was bound with a blood pact to the Devil, KAINEN. Little did she know, the mage harbors a secret love for her that will only complicate her training for the upcoming apocalyptic battle. This sets up the most unusual love triangle of all space and time.