Hakushaku to Yousei London Hashi ni Hoshi wa Tomoru

Edgar has entrusted Lydia to Kelpie in order to protect her from the Prince. Although she promised to marry Edgar just before they parted, the water horse’s magic has robbed her of her memories of him. Meanwhile, a mysterious disease spreads through London’s East End. It is said that the only way to survive it is to board the Ark, a mysterious vessel moored near London Bridge. Edgar cannot help but suspect the Prince’s involvement... (Source: J-Novel Club)