Well, no, I'm not inventing new ways to inflict blunt trauma. I can't top the millions of years of research that has already gone into that.
Actually, I haven't even gotten to the war yet; my little kingdom hosts a yearly jousting tournament in the late autumn. The national championships, as it were. Excellent chance to inflict blunt trauma on each other.
Here are some of the things I found myself needing to invent ways to treat in my fantasy world:
Hemothorax: Hemo = blood, thorax = chest area. When ribs break, they can tear the surrounding tissues and cause bleeding. This can happen without breaking the skin, so the blood has nowhere to go and builds up between the layers of tissue. (Your organs have been individually wrapped for your protection. And also bundled in groups. These wrapping layers have a bunch of names and lie directly on top of each other, but are not necessarily connected to each other... so blood and sometimes air (that would be pneumothorax) can get in between.)
Your lungs are fragile things, and a growing lump of blood in your chest cavity cramps their style. You breathe faster to try to get more air, your heart speeds up, you show signs of suffocation (turning blue, cool skin, etc.) This can be fatal.
Treatment: Cut the flesh to let the blood drain out and stop the bleeding. You can find the place to cut by tapping and listening to the hollowness of the chest IIRC. Careful of clots, they will clog the incision.
Ruptured spleen: Nowadays this is caused by car accidents or getting tackled by a linebacker. I'm willing to bet that falling off your horse at full tilt would be similar to a car accident -- this would not be as common an injury as broken bones, but it could happen.
Your spleen filters your blood supply, and thus a lot of blood moves through it. If it's smashed, it's going to bleed a lot and that blood will build up inside the abdominal cavity. This can be fatal due to blood loss. If the bleeding stops, you're still going to be swollen and in a lot of pain while your body clears the mess and tries to repair the damage.
Nowadays, surgeons go in and remove the spleen. You can manage without it. But in primitive conditions, that's not an option unless you have magic on hand. Herbal painkillers might take the edge off, but I'm thinking that for pain management you'd need something opium-based.
Flail chest: This is another unusual injury, but it looks dramatic. Flail chest happens when three or more of the ribs have been completely broken in two places so that there's an area that is not anchored to the rib cage at all. Just the skin and muscle holding it together.
This looks scary because the act of breathing is going to cause this area to move in the "wrong" direction with respect to the ribcage -- it moves outward when you exhale and inward when you inhale.
This really is scary because if you got hit that badly there is probably all kinds of internal damage to your lungs as well. The flail chest itself is not the life-threatening part of the problem -- it can be stabilized with bandages and will heal itself as any broken ribs will. Meanwhile, the healer needs to address any hemothorax, pneumothorax, bruising and bleeding in the lungs themselves, and so forth. The first two can be treated without magical intervention. The second two... maybe one could stitch a wounded lung even in primitive conditions. Infection is kinda guaranteed. It's a tough call.
What sorts of injuries do you inflict on your characters?