Here is an overview of the downloadable content, via its Steam page:

About

In a conflict that pre-dates the formation of the Star League, Join Colonel Mansdottir and the people of Rasalhague in a fight for freedom and independence from the Great Houses, in a new 12 mission quest line. “Rise of Rasalhague” adds the Crusader ‘Mech chassis with 10 variants, the new Rival Mercenaries feature allowing you to interact with numerous other mercenary companies and the infamous Bounty Hunter.

Key Features

• “Rise of Rasalhague” Campaign
  • Witness first-hand the rise of the Free Rasalhague Republic with a new 12 mission quest line.
• New Crusader Class
  • “Rise of Rasalhague” adds the Crusader ‘Mech chassis with 11 variants, including the Hero “Crael” variant with melee weapon capabilities.
• The Inner Sphere is More Alive than Ever
  • Rival mercenary companies are now operating in the Inner Sphere.
  • Working in areas garrisoned by Rival mercenary companies can lead to friendly or hostile interventions during missions.
  • The infamous Bounty Hunter will pay for intel on Rival mercenaries, which can lead to valuable rewards.
  • Experience exciting and dynamic missions in the new Infiltration game mode.
• Free Game Update
  • Various significant improvements to AI performance and behavior.
  • Redesigned ‘Mech Hanger allowing you to expand your active ‘Mech roster.
  • Many Quality of Life improvements making gameplay experience even more enjoyable.

MechWarrior 5: Mercenaries is available now for PlayStation 5, Xbox Series, PlayStation 4, Xbox One, and PC via Steam, Epic Games Store, and GOG.

Visit the “Rise of Rasalhague” official website here.