A Very Safe Supplement

A Very Safe Supplement is the twelfth chapter of the Owari no Seraph manga.

Short Summary
Yūichirō and his friends make their way to Shinjuku in time for the city's defense against vampires. Meanwhile, Glen identifies the invasion force's leader, Felid Bathory, and proceeds to eliminate him with his forces. He fails to eliminate Ferid, and is instead brought to a brawl with Mikaela Hyakuya.

As Yūichirō and his friends make their way to assist Glen, Shinoa reminds them of using a drug that could help them increase their strength to fight vampires, but warns that an overdose could kill them when they use the drug. Amidst the ensuing struggle, Glen was overpowered by Mikaela and Felid (who interfered and struck Glen), and as he was about to take a second possibly lethal dose, Mika thrusts his sword at Glen. Immediately afterwards, Yuu saw the situation and charges at Mika with his sword, but at that moment both Yuu and Mika recognized one another.

Long Summary
The chapter opens with a flashback to four years ago. Mika and Yu are hanging out with the other children, and Mika asks Yu about escaping the vampire city. Yu tells Mika they would never get out, but Yu admits it would be cool if they and their family escaped. Much to Yu's confusion, Mika says that he will try harder after all.

In the present, Shinoa's squad is on the battlefield in a heavily damaged area with numerous corpses of both human and vampire soldiers scattered left and right. The squad slaughters vampires and heads off to meet with Guren at Shinjuku's 5th Street intersection.

Shinoa surprises the squad newbies with a "very first training session." She explains the concept of drugging up to drastically raise ones synchronization with a demon for 15 minutes. One tablet raises power to 150% while two tablets raise it to 180% but may kill the user by sending him/her into shock. Three tablets will rupture all of the user's internal organs. After the 15 minutes are up, the synchronization drops to zero, and the user is utterly defenseless. In order to avoid this drawback, the soldiers carry a timer. This timer goes off after 13 minutes, signaling for the soldier to flee to safety before becoming extremely weak. The supplement requires 10 seconds to become active, which is why they did not have time to use them when they encountered Thirteenth Progenitor Crowley Eusford in chapter 10.

The side effects are extreme, leading Shinoa to joke, "The Demon Army itself one hundred percent guarantees that these supplements are positively riddled with harmful side effects." This unnerves the boys, but they each take a packet of the pills anyway.

Mitsuba and Shinoa explain that the ranking officer at the battlefield gives the orders to take these drugs, but most prefer to avoid them unless necessary.

At the 5th Street intersection, Glen and Mika clash. Felid warns Mika about his overwhelming arrogance, but Guren and his squad already managed to lure Mika into a trap. Glen nearly manages to stab Mika from behind when Ferid interferes, backhanding Glen and sending him flying while claiming that Mika owes him one. Mika rejects Ferid's request to fight together, and he orders his sword to drink more of his blood.

Glen contemplates taking a second tablet. Mika asks if he's done talking, and Glen asks if he can have another 20 seconds to take his medicine. Mika answers by attacking with greater strength and speed than before. As he prepares to make the finishing blow, Shinoa's squad appears and takes their drugs.

Mika stabs Glen through the chest. Yu arrives to protect Glen and promptly stabs Mika through the chest. Yu curses Mika, saying, "Die, Bloodsucker... Huh?" when their eyes meet.