From their website:
Back in July, Fantasy Flight Games announced Banners of War, an upcoming expansion for Runewars that builds on the strategy and diplomacy of the war for the dragon runes. In our first preview, we looked at the “Commanders of the Battlefield” variant, which elevates heroes to a new level of strategic importance. We also began to see how Banners of War’s modular design lets players incorporate only the variants they choose, resulting in a tailor-made Runewars experience. Next, we looked at the increased importance of cities established by the “Rise of the Free Cities” variant and the Lost City mechanic.
Today, we’ll look at another optional rule: Development cards. These versatile cards represent advanced spells or technologies researched by a faction, and they confer all manner of special advantages to their owner.