A militiary soldier are persons in the Unsub series who are, due some painful or other mysterious operation, a very powerful Slidderich like wizard. The first official Militairy Soldier was Antonin Dolohov, who apparently accepted an operation to transform him from a Death Eater to a Militairy Soldier. A militairy soldier is actually a more powerful Death Eater.
A militairy soldier has just a normal appearance like a human person, he only, almost does not talk, and do not wear black masks or clothes, because they don't need them. However, most Death Eaters dind't transform in Death Eaters because most Death Eaters where killed by the Bombers in their last stand.
FAMOUS MILITAIRY SOLDIERS:
Antonin Dolohov. (transformed in order to help Voldemort).
Fred Weasley (transformed after his apparent death in a battle).
Shawnee McGee (transformed in carbon monoxide accident in her hotel room in the Leaky Cauldron).
Grindelwald (transformed after Voldemort was killed).
Knowle Rohe (reason of transform not revealed).
Yeksi (reason of transform not revealed).
Keesie (presumed to have been a militairy soldier, however he was normally killed by the Avada Kedrava curse).
Militairy Soldiers have opportunities for new normal curses, one of them named Capaxti and Pai. They have also a curse that causes a so called coma. Here is a list of curses that they are able to use as militairy soldier:
Capaxti (causes a wizard or a creature to break a body part)
Pai (a better curse which has a brutally more effect then Rictesempra).
Je Souhaite (causes a "dark wish" to come out).
Aktivita (causes a coma in the victim).
Dea (a new curse which causes deaths, however the curse is more survival then Avada Kedavra).
A militairy soldier also has immunity for all curses in the Wizard universe, even for the three unforgivable curses. They also have powers without a wand, which they use to come out of their hand, like fire, black smoke, the language of snakes and other unexplained powers.
It is not known how Antonin Dolochov had been transformed, together with Grindelwald, Knowle Rohe and Yeksi. However, Fred Weasley was found murdered, however still alive, with his body fulled with scars and almost unreconizable. However, suddenly suffered an ecdysis and transformed into the normal Fred Weasley, but actually not really normal he had become a militairy soldier. In the season 17 premiere it was shown how Fred's new powers where and that he had really changed. The new fred weasley was born... However, what happened with him after his disappearance is currently unknown. It had been revealed that they also had to try to do this with Guus and Puck after they where captured, but it went wrong with the same test and fully recovered.
However, it had been revealed there where more tests for transforming. The young female wizard Shawnee McGee had a carbon monoxide accident at the Leaky Cauldron and due to some kind of pioson with the monoxide she changed into a militairy soldier. She told this to Potter and Guus.
In the season 17 finale, a badly disfigured boy arrived at the Unsub team who said he was a wizard from a land far away from here. When Guus and Potter examined his apparent injuries, he revealed that he was kept in captivity in a steaming hot hole, which burned his entire body, while mysterious dark doctors saw him suffering. However, the attempt went wrong and the boy, who was actually Draco Malfoy, went free.