Arkham Light with Cthulhu Dark
You have a red Harm Die. Like the Insanity Die, it starts at 1. Every time you’re injured, roll it: if you get above your current Harm, increase your Harm by 1.
When your Harm gets to 6, you die.
When you’re prepared to risk injury to succeed (and when this makes sense), add your Harm die to success rolls. If it rolls high, make a Harm roll, as above. (You can’t do this if you’re also adding your Insanity die.)
When your Harm reaches 5, you can reduce it by seeking strength from the Mythos. This includes: staring into the Yellow Sign, taking the Oaths of the Deep Ones, eating a piece of Mi-Go in the hope of gaining its strength.
When you seek strength from the Mythos, roll your Harm die. If you get your Harm or under, reduce your Harm by one. Simultaneously, make an Insanity roll: if you get your Insanity or over, increase your Insanity by one.
Once your Harm goes below 5, you can continue seeking strength from the Mythos. And, each time you successfully seek strength, explain how your body deforms.