I sometimes brew potions because I want to level up alchemy but I rarely actually use them. I have some fortify alchemy gear I probably wasn't wearing and no skill points in alchemy. So I was searching through my inventory and found "POISON OF DAMAGE HEALTH" weight 0.5 value 636 "Causes 928 points of poison damage. Decrease the target's Magicka regeneration by 100% for 23 seconds." I'm wondering what's the recipe for this so I can make more? Thanks.
It's hard to be completely certain... however... one of the most likely explanations is that you previously had gotten the Jarrin Root as part of a Dark Brotherhood quest. This unique (!!!) alchemical ingredient significantly improves the power of the damage health poison.
Unfortunately, if you have not completed that quest, you will have to deal with your target in a different manner.
If you're on PC and you're into mods, you can consider getting this mod to get more Jarrin Root: