Question: Custom weapons that only deal damage of a specific type
 

I'm trying to make a custom magic weapon changes all damage dealt to untyped damage, similar to how a merciful weapon changes the damage dealt to non-lethal damage.

How would I go about doing this? I've already tried using
perform parent.assign[wSpecial.Untyped]
but this just comes back with an error.

Similarly, it is possible to do this with other damage types to make weapons that only deal, for example, electrity or fire or divine damage?