The Flash Gordon actor will provide multiplayer voiceovers as Richard Neville, a role similar to Blessed's Richard IV in Blackadder in 1983.
Executive producer Gordon Van Dyke explained that the idea came from developer Fatshark, who wanted to redo the trailer for its upcoming re-release of the medieval combat game War of the Roses: Kingmaker.
"[We] were talking about remaking the launch trailer, and instead about talking about what makes the perfect weapon, which was the weapon of that one, is what makes the perfect king?" he told press including Digital Spy at a behind-closed-doors briefing.
"We should have someone playing the role of Richard Neville, and I want to get someone totally different to what we got before. I wanted to do something really impressive as this is a different trailer.
"I had this real crazy, bizarre idea that Brian Blessed should do that. And then we started thinking about it, and thought it would be so awesome if we could pull this off."
While Blessed was with them to record the yet-to-be-released trailer, they also asked him to supply multiplayer voiceovers for the game.
Blessed recorded around 350 lines of dialogue during a four-hour session with the team.
The DLC will release between the end of February or early March for an estimated $4.99 (£3).
Gallery: View images of the upcoming 'Outside the Law' update:
Copyright: Paradox InteractiveIt was one of many announcements for War of the Roses at Paradox Convention 2013.
The Kingmaker edition releasing late March adds new Assault and Assault Gate modes, Deluxe Edition DLC and support for game pad and Steam Big Picture mode.
Existing owners of War of the Roses will be upgraded to Kingmaker at no cost.
A free version of the game, described by Van Dyke as an "interesting experiment", allows consumers to play with default classes and modes alongside paying consumers from February 6.
Finally, the fourth free update to the game, 'Outside the Law', will add a new map later this month.
War of the Roses launched on PC last October.
Watch the release trailer for War of the Roses below: