Delta 67 is a substance obtained from the blood of Diva. If it is supplied to human bloodstream and can mutate them progressively into Chiropteran. The Delta 67 Project began in Russia, was perfected in Germany and was later completed in 1957 in the Vietnam War under the direction of Cinq Flèches and the U.S. government. The later years were spent to modify its spread and raise the chances of its effect.
When Saya went to Vietnam for the second time she discovers that there was testing going on in Karl's mansion. The Delta 67 was being tested using children, like what happened with her father at the beginning of the story. In the final episodes Julia discovers that the Delta 67 effect is triggered by hearing Diva singing live. Her voice coupled with the distributed Cinq Flèches Gold with this substance hidden in it, prompts many people to be potentially at risk of being turned to Chiropterans.