So... I've recently been wondering about either a perk or a constant effect ability for NPCs only, which makes them take extra damage from silver weapons. Said NPCs would have otherwise be nearly unstoppable, but their vulnerability to silver weapons would make them take double triple or maybe even 10 times more damage or something. With the silver weapons in this case, playing a similar role to kryptonite against powerful enemies. I being no professional with the creation kit though, have no idea HOW to create a weakness to silver weapons.

I also googled something about EnchSilverSwordFFContact which might cause 20+ damage vs undead ghost and vampires via acter type or faction, HOWEVER, turning the value from 1.0 to OVER that, does NOT increase the bonus damage supposedly... So I can't just duplicate that, then add some of my created NPCs to a replacement faction.

And now I'm kind of stumped as to if, this is even possible.

Edit: Also... I only want it to be especially effective against certain NPCs I made, not NPCs from vanilla Skyrim or other peoples mods.

You could add a perk to your NPCs. Create a new perk, right click in the Perk Entries box, and choose "New." Select the "Entry Point" radio button. Choose "Mod Incoming Damage" from the dropdown box, "Multiply" in the function box, and then the amount you want to multiply damage by. Click the "Attacker Weapon" tab and add a new condition "HasKeyword WeapMaterialSilver == 1"

This should hopefully do the trick.