Morse code

Morse Code (モールス信号), stands as an ancient means of communication meticulously devised by Eta under the guidance of his mentor, Shadow. This pioneering creation marked the dawn of secure long-distance communication, intended primarily to establish discreet connections with members of Shadow Garden scattered across different kingdoms and regions. Its paramount purpose lies in providing an exceptionally secure and covert means of correspondence for the leaders and adherents of the Cult of Diabolos.

History
TBA

Functioning
TBA

Practice and study
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Weaknesses
The only weakness of Morse code, like any other hidden message, is that if someone outside manages to decode it, it becomes useless, but (to this day), not even the Cult of Diablos itself knows about this method of communication, since it is means, it is from Earth, and not from the other world.

Trivia

 * Morse code, at a real level, was developed to use the telegraph, a device also devised by Samuel Morse in 1832.
 * Communication by Morse code is not related or mentioned by the Light Novel, Web Novel, or manga, it only appears in the anime and Game.
 * It is expected that the medium of how Cid taught him, this means of communication, taught by Cid Kagenou, to his Shadow Garden Girls (especially Eta), will be revealed in the "Chronicles of the 7 Shadows" of the history of the Game. (until now, it has not been revealed).
 * It is unknown, from what point in Cid Kagenou's life, did he have knowledge or learn about Morse code, (or even know how to use it) in his other life.
 * It is unknown, from what point in Cid Kagenou's life, did he teach this method of communication to Eta, while she and the other "Seven Shadows" were trained by him, before the rescue of Claire Kagenou.
 * According to the understanding of Eta and the other members of the "Seven Shadows", such knowledge is due to the "Wisdom of Shadows" of their lord Shadow, without knowing that he was not the one who invented it.
 * Eta is not the only one who knows how to use Morse code, in Shadow Garden, it is understood, Eta has a group of Girls with her, who also knows how to use it, (It is unknown if the other "Seven Shadows" know how exercise it).
 * The morse code that Eta sends out at the end of final episode anime season 1 actually spelled out "To be continued".