Hello, I'm working on a custom race for a homebrew setting, fey progenitors of the elves who have mostly withdrawn from the material plane since those who later became known as "drow" made sure that there are plenty bad people with cold iron blades around. So far the only way to depict that was giving them DR/cold iron which is expensive and makes them look bad. (Really, they leave the material plane because of weapons who hurt them like normal weapons hurt every other living being? D'awww let's play the world's saddest song on the world's smallest violin.)
Now the mythic playtest rules have exactly what I would need.
Material Weakness: Select cold iron, silver, or wood.
Weapons made primarily from the selected material
automatically confirm all critical hits against you and
the critical multiplier is increased by 1, to a maximum
of ×4. If you ever gain damage reduction, no matter
the source, weapons made primarily from the selected
material bypass that damage reduction.
I'm a bit unsure how the race point cost should look. The closest thing would probably be elemental vulnerability a -2 RP, but the mythic equivalent looks a lot worse, so I'm thinking about -3 or even -4....