WARNING: The following contains spoilers for Marvel's Black Widow, now playing in theaters and on Disney+ Premier Access.
Despite being raised in the Black Widow program, Natasha and Yelena spend most of Black Widow trying to break the hold the Red Room has on its other agents across the world. Along the way, they end up dealing with the Red Room's secret weapon, and it's one of the MCU's most dangerous tools yet, as Melina's form of mind-control is hard to break and gives the Red Room an army of loyal Black Widows.
After Natasha escaped the grasp of the Red Room by defecting to S.H.I.E.L.D, she attempted to cement the change in her life by taking part in the assassination of Dreykov, the man in charge of the Red Room. This attack failed, leading the Red Room to institute methods to ensure their future agents didn't commit a similar betrayal. On top of the strenuous psychological conditioning already used on the Black Widow agents, the Red Room -- and Melina in particular -- developed a chemical that can induce a level of control over them. It takes a special pheromone to release the Widows from this, giving Dreykov a new level of power in the world and an army of completely loyal Black Widows.
Early in Black Widow, Yelena is exposed to the cure by accident -- targeting another Widow who'd been freed and was trying to transport the cure to a new location, and this Widow dies before she can achieve this. While there is a cure, Melina's mind control technique is quietly one of the most frightening weapons to be introduced into the MCU yet.
Perhaps the most horrifying aspect of this control is that the victims appear to be aware of what they're doing, yet they have no way to stop themselves. It can force those under control to complete assignments they'd otherwise resist, with one Widow even forced to end her own life rather than risk divulging information to Natasha and Yelena. Even as she raises her weapon to her head, the Widow whispers that she doesn't want to go through with it before being forced to kill herself.
With that kind of method at their disposal, the Red Room had been able to arrange an entire global network of agents, all capable of carrying out any malicious order they're given by their handlers. As Dreykov points out, this could have even resulted in him gaining his own agent within the Avengers organization if Natasha hadn't gotten the better of him.
It's a particularly dangerous weapon that could turn entire populations into tools, giving someone like Dreykov -- or worse -- the potential to earn themselves some serious power and authority in the world. In essence, it's a mind control technique that, if brought on to the larger public, could become one of Marvel's most potentially dangerous weapons.
Directed by Cate Shortland, Black Widow stars Scarlett Johansson as Natasha Romanoff/Black Widow, Florence Pugh as Yelena Belova, David Harbour as Alexei Shostakov/Red Guardian, O-T Fagbenle as Mason and Rachel Weisz as Melina Vostokoff, with William Hurt as Secretary of State Thaddeus Ross and Ray Winstone as General Dreykov. The film is playing in theaters and on Disney+ Premier Access.