Ryuu to Ryuugoroshi no Miko

A certain human country has become prosperous in exchange for sacrificing a maiden to a dragon. But with their progressing civilization, humanity then found the power to defend their country without having to rely to the dragon anymore. Feeling that the dragon has already became a relic of the past, they choose a priestess to kill the dragon... However, the dragon fell in love with the priestess at the first sight instead! (Source: AlphaPolis, translated)