A battle in Solium Infernum consists of three types of combat. First ranged combat happens. Then melee combat. Then infernal combat, which is basically the part where demons fling fire and brimstone at each other. A battle progresses through each of these types of combat in turn. Even when these values are modified by artifacts and praetors, the numbers are always visible to everyone. Since there's not much randomness involved, you can look at these numbers and tell exactly how well a unit is going to fare in battle.
But then there are the combat cards. Your archfiend can spend resources to craft face-down combat cards that change the numerical values. The power of these cards is based on the archfiend's wrath, a stat that governs his military ability. A low wrath means your card can only tweak one value a few points. But a high wrath means a single card can dramatically change up to three values in a battle. Beware the unpredictable nature of a wrathful archfiend!
Here's an example of a typical artifact: a +2 sword that adds two to your legion's melee value. Or a +4 bow that adds four to your legion's ranged value. But this is Hell. There are no typical artifacts in Hell. The +2 sword is instead a Rod of Pain. It adds two to your legion's melee value, but it also has a chance to randomly reduce all three of the target's combat values by one to six points for the duration of the battle. Ouch. The +4 bow is instead a Hellfire Ballista. It adds four to your legion's ranged value, but it also inflicts a sulpherous burn that doubles the damage inflicted when you score a hit in ranged combat. Almost everything you buy down here is going to do something funky to change the rules. Things in Hell get complicated quickly.