How do I make attackers take damage?

[Blobathehutt]

I need help coming up with something along the lines of the black blood potion from witcher 3. For those of you who do not what that is here you go "Witcher's blood injures Vampires and Necrophages when they wound him." So to put it simply I need some way to make it so when an enemy damages you they take damage as well. I know you can do something like this with barrier but that is only if you block their damage this is if they deal damage.

[Malancthon]

For the xenomorphs, I used Break for their acidic blood, to represent that they were critically damaged to cause the blood attack.

However, you'll probably want to use Requirement to make it activate whenever your character gets attacked. I'd say it's -5 if it's just when you get attacked, -10 if it requires a common type of enemy to attack, -15 if the enemy type is rare, or -20 if it's a specific enemy.

Since Requirement, well, requires you to get attacked first, you don't really need any other restrictions for the Attack to activate.

[Blobathehutt]

So maybe something like this?

Blackblood
Unique perk (Reflex attack) +10, Requirement -10 (Must take melee damage from undead attacker)

Unique perk (Reflex attack) This attack deals damage as a free action.

I know it is a little strong but I'm not so worried about its power just if you guys think it will work.

[Malancthon]

That looks fine. I'd say the unique perk isn't really necessary since the requirement implies that it happens when you get hit, but if you're concerned about balanced, then that unique should work well.

[Clay]

This might be going a little bit outside the box, but you could use Barrier to achieve this effect by giving it Effective. I realize Barrier calls to mind energy fields and the like, but there's no reason it can't be much less flashy and just represent the defender's skin/blood.

[Blobathehutt]

This might be going a little bit outside the box, but you could use Barrier to achieve this effect by giving it Effective. I realize Barrier calls to mind energy fields and the like, but there's no reason it can't be much less flashy and just represent the defender's skin/blood.

That is true never thought about it like that.