Mannequin Soldiers

Towards the conclusion of Fullmetal Alchemist (and its anime adaptation Fullmetal Alchemist: Brotherhood) as Father's plot is coming to fruition, a corrupt Central general activates the Mannequin Soldiers (also known as "The Immortal Legion") to attack the Briggs forces. However, the Mannequin Soldiers are not under their control, instead attacking and devouring all humans in their path.

Role
Once Olivier Mira Armstrong is promoted to Central following her murder of Lieutenant General Raven, she is shown the slumbering Mannequin Soldiers beneath Central Headquarters by General Gardner. The corrupt general reveals that they are "The Immortal Legion" that will serve as their all-powerful army. (Thus establishing that the creation of humans isn't forbidden by the government due to ethics, but so as to not create competition/enemy forces.) It is shown that they are mannequins that, once injected with liquid Philosopher's Stones, will attack without the fear of death.

During the attack, one of the corrupt Generals activates the Mannequin Soldiers despite a leading scientist's protests that they aren't ready yet. One of the Mannequin Soldiers utters "Papa" with which the corrupt General attempts to manipulate it by acting as its father. Instead, the General and scientist are eaten alive by the Mannequin Soldiers that move out into the Headquarters, killing and eating soldiers along the way.

Several are confronted by Ed, Scar, and the chimeras Darius, Zampano, and Jerso. Despite their best efforts, the Mannequin Soldiers prove immune to the effects of most attacks and bullets, only contorting a bit or oozing green blood as a response.