This mission serves as a tutorial for army recruitment and fortress conquest. Begin it by going to the arena and watching your general-to-be crush another captain.
Now you have to beat down the survivor, Bruz, in order to crush his will and bend him to your service. Typical good guy stuff. There's an objective to SPACEBAR-evade him three times, and it's easy to get in the regular course of combat. If you ever get pressed, the tall poles here make dandy hidey-holes, and Bird of Prey launching platforms.
When the captain is wounded enough, you can use Dominate to recruit him (in this story mission, it'll work even if Talion is of a lower level than Bruz).
This ends the mission, but Bruz will continue to pop up throughout the level to offer you advice, wisdom, and tutorials on capturing other captains and organizing your forces.