With this perk, when person X's health drops below 25%, you gain +5% damage. When YOUR health drops below 25%, person X gains the same bonus.


All right, I got the damage modifier done, but I can't figure the conditions.
The latter, NPC getting the bonus, is not absolutely necessary if it can't be done with reasonable modding. I can live without it.

It can be done if 'perons X' is one unique NPC. Otherwise I could not help you.
Perhaps some more detailed info on what you want.

Yes it's a unique NPC. I guess I'll be using the REF ID then?

Yes, you give the NPC a Reference ID.
Then in the conditions, you can set the reference in the 'run on' drop down list. And you can use 'GetHealthPercentage' for the condition function.
If your checking against the player, choose the player as the reference. (the cell does not matter with the player)