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What do they do? Can I have a rulebook referance page please?

They interact with certain other special rules such as Ethereal or the Wood Elf Tree Spirits rule. They don't do anything on their own.

Yah, they are just to counter Ethereal. Both special rules are under...special rules. Around 76 I think (that's my go-to page when looking up specials).

"Q: What are ‘magical attacks’? (p68)
A: All attacks made by spells and magic items are considered to
be magical attacks, as are all attacks that are specifically noted
as being magical attacks. Shots fired from magical items are
also considered to be magical attacks, unless their description
specifically states otherwise. Hits inflicted by rolls on the
Miscast table are treated as magical attacks"

From the BRB FAQ. I don't know about the rulebook reference though.