Logos Mystery & Lost Master is the forty-eigth chapter of the Trinity Seven manga series, released on November 8, 2014 in the Monthly Dragon Ague December 2014 issue, and later compiled in the eleventh volume on March 25, 2015.
After binding the knight with her magic, Selina declares to a grateful Lieselotte that she will save her sister. However, the knight easily breaks the bindings, but having already foreseen that possibility, Selina simply summons additional binds that completely envelops her opponent, much to Lieselotte's surprise. Selina explains that her grimoire Archytas Replicator can calculate and store different patterns of defeat, although she does not necessarily have a solution to those patterns. Nevertheless, Lieselotte becomes proud of her younger sister, which Selina also becomes estatic at being recognized. As the knight continues to struggle, the two decide to fight together despite their adversary's similar strength to Arata. Lieselotte assures her twin that as soon as the knight liberates itself, they will perform a spell that requires two Logos Art masters. Upon the knight breaking free, the sisters then invoke their spell to entrap their opponent within a space time prison. The two are victorious, but fatigued, while Selina in particular becomes nude after exhausting all of her magic. After giving her sister a blazer, Lieselotte nontheless notes that their enemy was powerful enough to force them to use their combined magic, wondering if she will meet Arata the next day.
Elsewhere outside, a confused Arata suddenly awakens, which Anastasia-L explains that he was having a nightmare. He recalls that he was fighting against Lieselotte in the infirmary before being forced into another dimension. Suspicious, Arata then questions Ana which she reveals that the Demon Lord is an existense similar to her spirits. As such, she desires to rescue him and the world, as well as revealing the captured Akio and Levi much to his surprise. Because he will eventually awaken as the Demon Lord to subjugate the Trinity Seven and the world, Anna declares that she will save him before causing his arm to transform. Embracing him, the Saint of Resurrection states that he should escape his destiny by going into Demise along with his friends, invoking a spell that forcibly converts him into Astral Trinity. Ana tearfully apologizes to Arata, although he responds that she does not have to cry before disappearing into her. Surprised by his attitude, she comments that the nature of their encounter was regrettable, but nevertheless resolves to confront the person responsible for her situation, Lilith, as the previous Trinity Seven of Luxuria.
During Akarsha's Underground Magical Research Showdown, Lieselotte was overwhelmed by a mysterious knight figure until an awakened Selina rescued her.
In order to save her elder sister, Selina's camera transformed into the grimoire Archytas Replicator, allowing her to finally access her Magus Mode.
Arata awakens to find himself outside with Anastasia-L at night. The two had initially agreed to meet after the first round of matches in Chapter 43.
Ana reveals to Arata the crystals containing Akio and Levi. The two Trinity Seven were originally captured after being defeated by Arsha in the previous chapter.
In order to defeat the knight, Lieselotte and Selina invoke their Numerals, 66 and 6 respectively, to cast their foe into a time prison. Together, the numbers form 666 which in Christian mythology is often associated with the devil, the Anti-Christ or the Beast from Revelations.