My Ancients & Aliens campaign has a lot of house rules and idiosyncrasies, maybe too many for some. I can see new players struggle a bit with all the unfamiliar bits and bobs, especially during character creation. Maybe I should have started the PCs in opposition with civilization so that the players could all play the more familiar outcast races (elves, dwarves, escaped humans, ect.), but instead I threw them into the deep end with all the crazy Anunnaki slave races. I'd defiantly go that route if I was to use this setting for a one shot.
Anyhow, one of the things that I'm happiest with is the way that combat is playing out during these tabletop games. I think I've struck a good balance between critical hits, fumbles, and the house rules; so I thought I'd go ahead and share all that in case someone else wants to give it a go.
Every natural 20 attack roll is a critical hit. That goes for PCs, monsters, whoever. Roll d100 on the Arduin Grimoire Critical Hit Table. That thing is wicked awesome.
Every natural 1 attack roll is a fumble. That goes for PCs, monsters, whoever. Roll a d6 on this fumble chart to see what happens.
1. Awfully Unforgettable Fumble - Roll d100 on the chart
2. Off Balance: -2 To-Hit penalty for your next attack roll
3. Dropped Guard: +2 To-Hit for next attack roll against you
4. Trip: start next round on the ground
5. Drop Weapon/Disarmed: inconvenient but retrievable
6. Recover Beautifully: no effect
House Rules and Stuff
That critical hit table is rough. One of the results reads [chest Heart Pierced, immediate death 1d10] the numbers at the end there being the bonus damage rolled in addition to "immediate death" and normal weapon damage. As you might imagine, five percent chance per attack makes for a LOT of gruesome critical hits. However, there are a few house rules to help prevent some of these horrific wounds.
Shields - In my game Shields shall be Splintered, so anyone can sacrifice their shield in order turn a hit into a miss (even a crit).
Protective Gear - Helmets, metal collars, and other such gear will become ruined when they prevent a critical that targets the protected body part. You'll probably still take the normal damage though.
Be Tough - Halflingons are tough as nails. They've got redundant organ systems, so stabbing one in the heart might only piss the dude off. They ignore any mention of "death" on the crit chart, but still take the bonus damage. Also, certain mutants will have crit preventing mutations that only apply to certain locations. You can't get decapitated if one of your mutations is "Headless", so you'd avoid the crit and the bonus damage but still take normal damage from a standard hit.
Oh wait, decapitated halflingons, hmmm... I hadn't really thought about it before. I suppose they must have a psychic connection to some kind of rudimentary lizard brain at the base of their spine. The headless body would still be able to move and attack, but I'd give it a -4 penalty for fighting blind unless they are holding their head with their off hand. Blood loss and starvation would eventually become an issue though, so they should probably seek medical attention after the fight.
Be Impossible - We haven't really come across this yet during play. If the critical result seems impossible due to the size difference between combatants or because of the weaponry involved then I'll just make up an effect for your critical. No matter how good you are with a whip I'm fairly certain that it won't stab someone in the heart, but a whip might disarm or trip a person. If I can't think of any cool effect then you'll get some bonus damage, roll one extra damage die.
So, without that splintered shields house rule I think the Arduin Criticals would be too lethal, too often. I'd probably want to embed it into a d6 chart, similar to how I have the "awfully unforgettable fumbles" tucked within that d6 fumble chart so that the weird and crazy results are a bit more rare. Maybe something like this...
1. Wasted Opportunity: no effect
2. Combat Maneuver: Normal damage and target rolls a fumble
3. Solid Strike: double damage
4. Attack of Opportunity: Extra attack +4 To-Hit vs. any opponent within reach
5. Dodge Offensively: Next attack against you misses and targets a nearby enemy instead
6. Arduin Critical Hit - Roll d100 on the chart