System Shock

One thing I liked about the old BattleTech table top game was filling in those little circles to account for lasers and missiles blasting away at mech armor until the volleys reached the soft center and started doing lasting damage. Felt dramatic to me. Combine that with in D&D 5e the idea of taking a nasty war hammer blow to the head with no chance of your bell being rung per the standard rules. Doesn't seem right. To model being susceptible to notable and sustained damage I use a house system shock rule. Notably vicious blows should impair the target beyond simple damage because bones break and brains get concussions before the target drops dead. My first thought was that as part of a critical hit you would have a system shock. That thematically felt right but after thinking it through it is flawed. You should not have a level one wizard wielding a dagger run up to adult dragon and in one lucky jab possibly give the dragon a concussion. Too silly. So instead notably vicious is measured by amount of damage from a single blow. A single blow that does more than half of the targets remaining hit points feels like a good amount. Remaining hit points is key here. As any target is beaten down it becomes more and more susceptible to additional injury. Imagine Rocky against Drago in the final rounds with face a swollen mess, barely able to stand, and just one more solid blow could be a knockout. So each time more than half of target's remaining hit points are inflicted by a single blow the attacker rolls 1d12 and consults the table below.

So notice that the medicine check has to be by someone that is proficient in the medicine skill. It doesn't make sense to me that Bob the Ranger who doesn't know a contusion from a concussion can just have a go at a dislocated shoulder and huzzah I happened to roll a seventeen.