Go to Static Objects > Technical > black_plane_black_1
And put thoose behind, if you are making a scenery like me now and dont have any plan that the player is going behind thoose walls, you want to consider perfomance istead of using walls which no one can see.
You want to face the black void against the monster, behind the black void is transparent for the player to see through, useful when working with windows or anything else.
PS: I am testing this right now as i write this, just figure something out if it doesnt work, I''ve scaled mine bigger and wider. - If it doesnt work maybe primitive will do the case with the less demanded texture. Primitives are also transparent from one side
EDIT: It works!