One of the most popular house rules that I've used over the last 4 years or so is a chart for Dynamic Critical Hits. It was born in the first days of 4th edition, when I was missing the old school, anything-can-happen, narrative-driven combat of my second edition days. I didn't want anything quite as complicated as this, but I did want to employ a method of making combat dangerous, with consequences that affected the story.
The result is this.
Here's how it works.
When a player rolls a critical hit, immediately roll a d10 and a d20 and consult the Dynamic Critical Hits chart. The d10 will determine the affected area of the critical hit, and the d20 will determine the critical effects.
The affected areas on the chart account for humanoids, beasts, and other creatures. The critical hit might affect a wing, limb, appendage, or other body part, depending on the nature of the creature. Remember that you as the GM have final say. If it makes more sense for the critical hit to strike a different area, you can choose a result rather than have the player roll it.
The critical effects are cumulative, meaning that whatever result the player rolls includes all the effects for lower rolls as well. The higher the roll, the more critical effects there are. The effects are as follows.
1-5: Roll damage dice twice. This effect is identical to the normal effect of a critical hit.
6-10: Knock target prone. In addition to the extra damage, the target is knocked over.
11-14: Target’s speed is 0 until end of its next turn. In addition to the previous two effects, the target’s speed is reduced to 0, which prevents it from standing up or crawling until its speed is restored at the end of its next turn.
15-16: Target stunned until end of its next turn. To all of the above is added the stunned condition, preventing the target from moving, taking actions or reactions, and speaking in full sentences. Attack rolls against it have advantage, and it automatically fails Strength and Dexterity saving throws.
17: Area gains an injury. In addition to all of the above effects, choose the attacker roll on the injury chart and apply the result to the victim. As the GM, you decide what additional effects might come into play once the area is injured, such as disadvantage on Wisdom (Perception) checks after losing an eye.
18: Roll damage dice a third time. Instead of rolling damage dice twice before adding modifiers, roll the dice three times.
19: Target cannot regain hit points until it recuperates. Record the number of hit points the target has remaining after the damage from the critical hit is applied. That number becomes the creatures hit point maximum (to a minimum of 1 hit point) until it spends three days recuperating as per the rules for downtime activity in the Player’s Handbook. The target can, however, gain temporary hit points.
In addition to this effect, the lingering injury inflicted becomes permanent. The attacker determines any specific flavor surrounding the injury—if the area is severed, crushed, gored, melted, frozen, or whatever terrible fate makes the most sense.
20: Instant death. This effect operates as if the target took damage that dropped it to 0 hit points, and the remaining damage was equal to its maximum hit point value. It dies instantly. You can ignore the affected area roll and simply have the player who rolled the critical hit narrate their kill.
The Injuries chart can be used in connection with the Dynamic Critical Hits option, or on its own. You might bestow an injury after a creature falls a long distance, takes damage from one source equal to half its hit point maximum, or drops to 0 hit points without being killed outright. You can roll an affected area on the Dynamic Critical Hits chart or choose one that makes sense.
Injuries can be healed with a regenerate spell, or a number of successful Wisdom (Medicine) checks equal to the d20 result you rolled when determining the injury. Each of these Wisdom (Medicine) checks has a DC of 15. The injured character must take a long rest between each check. If more than 30 hours pass between checks, or if the administering character fails a check, the process must start over from the beginning.
Injured Player Characters
While the prospect of having limbs and organs torn from their bodies may not seem immediately appealing, remember that receiving injuries is a great opportunity to roleplay and heighten the stakes of the adventure. As the GM, don’t force an injured player to sit out or feel useless. There’s no reason they can’t continue to participate in combat.
Be sure to frame follow-up scenes that emphasize the story ramifications of their injury. Who tends to the character’s wounds? How does this affect their relationship? How does the injury change the party’s plans? Answering these questions through roleplay will enhance the adventure and player investment.
Player character injuries are also a great hook for future adventures. Your wizard lost an eye in a fight with a zombie? Time to make plans for a new one—maybe a magical crystal that can peer into other planes of existence. Your fighter’s leg is lame? Begin a search for the best surgeon in the kingdom, perhaps even one depraved enough to graft an ape’s leg or kraken’s tentacle in its place. Losing a body part may close one or two doors, but it’s your job as the GM to make sure it opens far more.
So there you have it. I hope you can put this to good use. It's including on the Duct Tape DM Screen charts (combat panel) that I posted last week, but if you wanted a separate printable pdf, you can get the crit rules here.