Who plays with which heroes in the Inferno faction?

Who is the enemy, for example? A Tier 3 unit or a Necromancer? And what are the Imps for? And why wait when, if you take a quick Portal opening in logistics (I almost always do), units will only spend a quarter of their turn opening the portal, and the Unicorns and Cerberuses will most likely move again immediately, so we don't really lose anything by calling in reinforcements in a short game.