Sorry for all these threads lately, but I would like to know if it's possible for players to take magnified damage from all sources when they have a certain potion effect.
The answer is the same as with your other questions, of course it is possible. Why don't you try to implement it, instead of asking? Then if you get stuck, you can ask more specific questions here.
Assuming Forge hasn't changed much since 1.7.10, you should be able to use "events".
The documents Forge gives isn't much compared to the code they do give.