Acma:Game: Aratana Kagi

A 2 episode spin-off set after the drama and leading up to the movie.
“The new key” begins a few months after the collapse of Gungnir when Teruasa and Senya, who had been on a journey to collect the remaining “Devil’s Keys” return to Ui and Yuki. They learn that someone with a key is on the island where the final tournament was held. Without telling Yuki, Teruasa, Senya and Ui head back to the island. Inside the crumbling mansion, there was a shadow of someone—or something. After chasing the shadow into the mansion, they realize Senya has suddenly disappeared. Then, before the two of them appears a woman, Kizuki Mai, who is trembling and crying. However, the moment they call out to her, the woman bursts into loud laughter and summons the demon Elva with a “Devil’s Key” she pulls from her pocket. At that point, another man wearing white clothing appears. Who are these two people!? The life-or-death game for Teruasa and Ui begins once again.
(Source: Drama-Otaku)