Thrall/1.03

The Thrall is the primitive builder. He is cheap but has a poor build skill and combat.

Barbarians
Thrall serves as the basic builder of the Barbarians.

Benefits from the Training, Eagle Eye, Magical Tattoos, and Bloodlust researches.

Minotaurs
Thrall serves as the basic builder of the Minotaurs.

Benefits from the Training, and Eagle Eye researches.

Dark Elves
The Dark Elves can instantly recruit 10 Thralls for 250 Gold. While they can be used to build they are inferior to the Wisp so they are best used to fill mines. This aid Dark Elves in overcoming the limitation that their basic builders can't fill mines. Like all Elf races income from mine fillers is halves but it is still better then none. If Sacrifice has been researched they can be sacrificed for the possibility to summon a Daemon.

Benefits from the Black Ward research.

Undead
The Undead can instantly recruit 10 Thralls for 250 Gold. Since they have better speed then Zombies they can replacement for making buildings since they will reach they will reach the destination faster. They can be used for mine fillers but are not available until later in the game. They benefit from the Weaponsmith and Dark Mithril researches.

They can also be sacrificed en masse for their bones by pressing Crtl+D while they are selected. This will kill all of them simultaneously and their corpses would soon turn into a pile bones within a few seconds. With a whole field of bones at your disposal you can now raise a horde of skeletons in an instant by casting the "Call the Dead" spell from the center of the bone field or with the assistance of an evenly dispersed squad of liches if you've sacrificed more than 1 batch of thralls.

This is a good tactic since morphing skeletons to better units costs only either Metal or Crystals and thus it does not compete with the Gold resource which you will be using to summon more thralls to sacrifice. This process of making your army has an added bonus of being quicker, takes up less map space and is less costly overall than building a whole bunch of Gravestones but with the downside of being only available mid-game since the Cage is a Tier 3 building and that you have enough mines and stockpiled resources to do it. Plus it is heavy in micromanagement compared to just letting your Gravestones auto-produce skeletons for you.

Benefits from the Weaponsmith and Dark Mithril researches.

Quotes

 * Ready: "I serve only you, Master."
 * Select: "Ma... Master?" "Don't... Don't hurt me." "No, Master, no!"
 * Order: "As you wish." "Yes Master." "I'm so tired."
 * Deathblow: "I... I... I hit him!"
 * Can't build: "Master... It is too hard."
 * Build complete: "Please, let me rest."

Trivia
-Thralls were slaves or serfs in Scandinavian lands.