I put a very short section on Hit Dice as a Resource into the new v1.5 core book. It’s very short. And just today – I realised, too short.
The Hit Die
What’s in the core book is a fairly abstract and underdeveloped idea – so, use it at your own risk.
When you’re hit by something, instead of deducting hit points you roll your Hit Die like a Resource. If you roll a 1 or 2, the Hit Die drops a step. If you’re a Ruffian, you start with D6. Someone shoots you. Roll 3-6, you get a graze or something. Roll 1-2, you drop to a D4. Healing restores a Die step.
It works OK between fairly similar opponents, but when you start using big guns or monsters it makes less sense. If you battle a Shoggoth, something has to give because that Crush attack is not small beans.
Big Damage, Big Disadvantage
Therefore, I offer this suggestion as a general system – more damage has a greater chance of forcing a drop.
When hit with damage, compare the amount against the Hit Die of the character. The player then rolls the Hit Die with appropriate adjustment. A big creature means the possibility of rolling with Disadvantage – roll two dice and take the worst result – or Double Disadvantage – roll three dice and take the worst result.
This means that a Shoggoth will make a greater impact that a Cultist with a knife. Robust characters will get Advantage on their Hit Die roll if all they face is a knife, but everyone rolls with Double Disadvantage when subjected to a Shoggoth’s crush attack – unless the character with a D12 hit die can get behind the cover of a metal barrier (AP 9), which means they’ll only roll with Disadvantage.
The original example still holds – the Ruffian getting shot will still roll D6 as normal, as an average attack with a gun does 3 damage. That falls in the Normal range for making a Hit Die roll. A shot run, which might do 5 damage by an NPC, means Disadvantage.
Player vs Player
Really? In Cthulhu? Like you didn’t have enough enemies already.
You have a choice:
Roll the Damage: Roll as normal and compare against the table. If you roll well with a pistol, it could be bad for that Ruffian. Roll a 6 on D6 for the pistol damage, and the poor chump has to roll with Double Disadvantage.
Half the Die Type: Take the size of the Die and half it. A D6 attack does 3; a D10 attack does 5. If you’re rolling multiple dice for some reason, add all the Die sizes together and then add half the number of dice rolled.
For example, the player has found some hideous Mi-Go blasting gun that does 4D4 damage. If the GM chooses to use the fixed damage method, it inflicts ((4+4+4+4)/2) + 2 (half the number of dice) = 10 damage.
Please use with care. While I have used Hit Dice as Resource myself, I did so in adventures with relatively human-scaled opponents; therefore, this has not been tested at all — but, it will be now!