Cover page of this doujinshi. In the story, Remilia Scarlet and Vlad III Drăculea, the Prince of Wallachia, are the same person.
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The sample pages are here: [Conquest of Wallachia] / [Sultan Mehmet II]

Tags removed due to there being nothing in the artist-added Pixiv tags, the artist commentary, or the picture itself, to indicate that the hand behind Remi's left wrist is Flan's hand.