Monster only in darkness???

[anzki]

Well there is entity or static object(can't recall which ) that is in 'technical' section, that is invisible, but blocks all movement, and if I recall correctly, there was a one that blocks only monster movement. Perhaps this helps you

E: It is entity under technical section; there is two of them 'block_box' and 'block_nonchar'. And I think nonchar is what you are looking for.