Battle Thralls

You are an Evil Overlord who have just conquered another place, crushed the puny La Résistance, set up the proper measures to keep people in line and are ready to spill more blood for the blood god. The Les Collaborateurs may serve as the sergeant to this new army, but often it is headed by a Fantastic Racist who will rarely show much mercy for this battalion.

There are several regiments of the Battle Thralls:

Proud Warrior Race Guy: This branch consist of a group of people who believes might makes right, they will not question the leader's loyalty, believing that since they were beaten via trial by combat that they now accept him as the new leader. To them, serving the leader is the utmost highest honor this battle thrall can have and that he is the source of enlightenment amongst them. However, woe be unto thee if there are some warrior race guys who feel you have insulted their honor. They will gladly be allies/non-aggressive with the rebels to return their group to their glory. In battle, they are tough bastards with strategy to boot so if you face these guys: Good luck.

Barbarian Marauders: Brutish, wild and the dumbest beings able to read. The Barbarian Marauder is perfectly fine with serving under the new rule of terror, but he fights for one reason: to cause terror and bump off anyone the empire doesn't like for their own pleasure. Basically, sociopaths pointed in another direction. Their brutality is offset by their shortmindedness. If they don't get their hedonistic needs met, they will pick apart your empire bit by bit.

Engineers of Doom: Every army needs folks to keep the war machine well sharpened and often these engineers will do the work. They rarely take sides in the conflict, only working to make sure the war machine is running and will rarely fight with the rebels if needed. They are often the straight man of the Battle Thralls, being the voice of reason amongst the Chaotic and Lawful Evil groups from killing each other. If the opportunity is given to rebel, they will merely offer a token of assistance but will not directly help in sabotaging the war machines.

Enslaved Grunts: The lowest rank of Battle Thralls, basically rounded up and given a crude weapon and maybe a bomb. Their goal is to employ Zerg Rush Tactics against the enemy. They are regarded by both sides as target practice and friendly fire is often an issue. They rarely have the courage to stand up and fight against their overlords. If not "freed" by the rebels, they often realize that their lot in life is to die anyway and thus may sacrifice themselves for the emperor, no matter how hopeless the attack is.

Sneaky Mercs: Thieves, bandits and other lowlifes who are more willing to harm a helpless target but would run at the first sight of someone getting killed. Their role in combat is to act as the surgical knife of the lowlifes, stealing, robbing or causing acts of terror with minimum bloodshed. Or until they plant that bomb and get the fuck out of dodge. Have an egotistical complex due to the fact that The Empire would gladly allow lowlife such as them to exist when they should be put to the sword.

Alluring Amazon Slave Girls: The bedwarmers of the battle thrall army. To the Empire, they are a useful resource for keeping the ruling elite company during those cold nights after a hard day of plundering and pillaging, and not too bad as soldiers either. Mostly used to infiltrate resistance cells or the beds of defiant generals. They often become enamored with the main character if he is a dashing hero. Frequently one will personally fall in love with the hero and realize that her love for the man who is trying to reveal his secrets is genuine. The Vamp version would often have a tragic romance with the ruler, realizing that he can not truly love her.

Dogmatic Drones: Robot Mooks, Bug War, a Dogmatic Drone Army are pretty much nameless/faceless robots banded under one leader who gave birth to them and a purpose. Often in the Lawful Evil territory. They act as a deadlier version of a Zerg Rush. Unlike Grunts, they don't have rebellious tendencies but are prone to have "unacceptables" who have broken off with the main society and are pretty much now unleashing their anger in all directions.

Examples of Battle Thralls include:

Proud Warrior Race Guy: Mandalorian Clone Troopers in Star Wars (Some Clone Troopers who valued their roots then to the Republic and later the Empire).
Barbarian Marauders: Khorne Berserkers, they're more of a freelance group. If you don't give them enough victims or opportunities to fight, they will gladly turn on you.
Enslaved Grunts: The Kushan Empire in Berserk uses captured enemy soldiers as frontline Cannon Fodder: They'll get killed if they try to run away, while the enemy may have difficulty shooting their countrymen.
Star Control is the Trope Namer with the Hierarchy of Battle Thralls led by the Ur-Quan Kzer-Za, although you learn more about them in Star Control II.
- Proud Warrior Race Guy: The Thraddash. Possibly overlaps with the Barbarian Marauders (they're proud, but also dumb as bricks and unapologetically violent to a fault). The Yehat in Star Control II are a more straight example; being classic Proud Warrior Race Guys who serve the Ur-Quan Kzer-Za willingly under orders from their Veep-Neep Queen; in the first game, the Yehat were part of The Alliance of the Free Stars.
- Barbarian Marauders: The Ilwrath. They live for evil and became Battle Thralls to the Ur-Quan Kzer-Za solely for the purpose of obtaining sacrifices for their dark gods. The VUX are a bit of a subversion; they're intelligent and very eloquent, they're just narcissistic as heck and are heinously violently solely because they consider every non-VUX lifeform (humans especially) in the universe uglier than they are.
- Engineers of Doom: The Umgah. Pranksome geneticists that can be helpful if you can figure out why they're so quirky.
- Enslaved Grunts: The Spathi, a race of cephalopod cowards. The Mycon might also apply here, in the "living bomb" sense - they joined the Ur-Quan Slave Empire in order to gain new worlds to terraform by burying their children into its crust.
  - In general, Kzer-Za policy limits the slave part of it—a race becoming Battle Thralls is supposed to be a choice by that race (with the alternative being Slave Shielding), but the choice is final... even if an Umgah practical joke made you think you were choosing to be Slave Shielded.
- Dogmatic Drones: The Mycon are the unholy combination of Planet Eaters and a Horde of Alien Locusts.
In the Dungeons and Dragons Dark Sun setting, Enslaved Grunts come in both slave soldiers and gladiator varieties
Kull Warriors are Dogmatic Drones of the Super Soldier variety, and until a weapon specifically made to kill them quick was developed, also very hard to stop.
You can recruit such thralls in the first Medieval Total War. Specifically, the expansion pack, Viking Invasion.
- Still further back, in the Shogun: Total War expansion Mongol Invasion, the Mongol's basic, cheap infantry units are basically Korean captives / vassals pressed into service.
In Warriors Orochi, the Wei and Wu became Les Collaborateurs and Battle Thralls with Orochi
In Xenology, the Ethereals are hinted to have turned the Tau into Battle Thralls for the Eldar since their innate warp resistance meant they won't be controlled by chaos nor be wary of being manipulated by the Eldar.
Yuber from Suikoden is a mix of Barbarian Marauder (he destroyed a whole village because he was bored while waiting), Enslaved Grunt (he can be summoned and will work for the ones responsible for doing so.. at least until he's in danger) and Sneaky Merc (he actively seeks, craves and creates chaos and participates in most wars. However once he feels that he is in danger he will flee, usually by teleporting, and abandon the squad he was commanding and the battle he was fighting).
In One Piece, Hody Jones's New Fishman Pirates is partially comprised of human slaves fitted with explosive collars. If they do not fight, they are blown up.
In Code Geass Lloyd, "the earl of pudding" is an Engineer of Doom. Actually he is only interested in machines and doesn't really care much about humans.
In Belisarius Series the Rajputs and Kushans are Proud Warrior Race guys; both of them defected though the Kushans defected earlier(the Rajputs had to be left so that Belisarius would have someone decent to fight). These two are more Worthy Opponents then the rather grim jackboots described above; the Rajputs are aristocratic warriors and the Kushans tough professionals, not totally unlike Romans. The Ye-tai are barbarian marauders and the conscripted peasants are the grunts.
An army of Enslaved Grunts pops up late in Code Geass, controlled by Emperor Lelouch's Geass as part of a deliberate play at 0% Approval Rating right before pulling a Silent Scapegoat.
Halo's Covenant has most of these, with the Prophets in charge:
- The Elites (Sangheili) are the Proud Warrior Race Guys, of course. The Brutes (Jiralhanae) are the Barbarian Marauders. The Grunts (Unggoy) are, well, the Enslaved Grunts. The Jackals (Kig-Yar) are the Sneaky Mercs. The Drones (Yan-Me) are, you guessed it, the Dogmatic Drones. And the Engineers (Huragok) are, naturally, the Engineers of Doom.
- All that's missing is the Alluring Amazons.

Real Life

Near the end of its reign, Rome, the classic Vestigial Empire, was hiring a lot of barbarians, because its own army was depleted. They even hired The Huns at one point to take down the Burgundians. The Huns quickly figured out there was a lot more gold where their paycheck came from, and spent several years shaking down the Eastern Roman Empire, who had fairly unprofessional soldiers. Then Attila got cocky and took on the West. The West had trained soldiers and every Barbarian Tribe that Attila had ever wronged on their side. Not surprisingly, Rome won, and Attila died on his wedding night (not of a nosebleed as is commonly thought; his heavy drinking caused a hemorrhage in his esophagus, and killed him with a combination of choking on his own blood and shock) Now here's where Rome made a huge mistake: it let its Western army slack off, and discriminated heavily on the barbarians. Then, the barbarians all teamed up to form The Horde and carried out what is still considered to be the greatest ever Rape, Pillage and Burn ever conducted. The raid was so devastating that not only were barbarians all over Europe able to reclaim their land while the Romans just stood there staring at the bonfire they used to call 'home', but they actually destroyed 400 years worth of technology. Dark Age Europe ensued.
The elite Janissary forces of the old Ottoman Empire, later becoming a powerful military caste in their own right, began technically as Battle Thralls, mainly from the then newly-conquered territories such as Greece and Bulgaria. The Ottomans would acquire suitable young boys for the Sultan from those territories specifically for this purpose, making it also a Real Life case of TykeBombs.
- Well, almost a real life case of TykeBombs. They were taken back to Istanbul and given extensive training (in reading, history and religion as well as fighting), so it would sometimes be years before they'd see actual battle.