Mannequin Soldiers are a group of minor antagonists in Fullmetal Alchemist and its anime adaptation Fullmetal Alchemist: Brotherhood. They are a band of homunculi-esque bodies also known as "The Immortal Legion". Towards the conclusion of the series as Father's plot is coming to fruition, a corrupt Central general activates them to attack the Briggs forces. However, the Mannequin Soldiers are not under their control, instead attacking and devouring all humans in their path.
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, establishing that the creation of humans is not forbidden by the government due to a matter of 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 or harm. This also adds a tragic level to the beasts as they are fueled by victimized souls.
During the attack, one of the corrupt Generals activates the Mannequin Soldiers despite a leading scientist's protests that they are not 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. Armstrong and her brother Alex Louis battle several others with allied soldiers while also contending with Sloth.
Several are confronted by Ed, Scar, and the chimeras Darius, Zampano, and Jerso. Despite their best efforts, the Mannequin Soldiers prove to be immune to the effects of most attacks and bullets, only contorting a bit or oozing green blood as a response. The majority attacking the group are dealt with by Mustang's flame alchemy (crippling them in the manga, outright incinerating them in the anime). By the end, most are destroyed and it can be figured that they were all wiped out by the protagonists.
At one point, Envy is also shown absorbing several of the Mannequin Soldiers in order to gain power from their embedded Philosopher's Stone and recreate his body. This might also suggest another reason for their creation in Father's plot, providing backup Philosopher's Stones.