mob/shinigami/var
  spiritform=1//if zero then you are in your real body

turf
    Enter(mob/m)
        // We have to rewrite the Enter() proc to handle
        // this.

        if(density || loc.density)
            // If the turf, or the /area the turf is in, are
            // dense, don't allow entry.
            return 0

        for(var/atom/movable/n in contents)
            // Look through the other things in the turf's
            // contents.

            if(n.density)
                // If they're dense, then don't bother
                // checking if the user can see them. Just
                // block them from entering the turf.
                return 0

            else if(m.SeeCheck(n))
                // If they're not dense, then check if
                // the thing trying to enter the turf can
                // see them. If they can, then block the
                // player from entering the turf.
                return 0

        // If there is nothing else dense on the turf, and
        // the player can't see anything, then allow them to
        // enter.
        return 1

mob
    var
        list/can_see // This is a list of things the player
                     // can see.

    proc
        AllowSee(mob/m)
            // This is the proc that allows the player to 
            // see certain things.
            if(!can_see)
                // If the list hasn't been created, create
                // it now.
                can_see = new

            can_see += m

            // There are a couple ways you can handle
            // actually being able to see or not see mobs
            // and objs. I won't handle this here, as that's
            // beyond the scope of your question.

        DisallowSee(mob/m)
            can_see -= m
            if(!can_see.len)
                // If there's nothing else in the list,
                // delete it to free up resources.
                del can_see

            // Same as above, this is where you'd  write the
            // stuff to handle removing the thing from the
            // player's view.

        SeeCheck(atom/a)
            // If the thing is in the player's can_see list,
            // then return 1, meaning they're able to see
            // them.
            return (a in can_see)

atom
    movable
        proc
            //analogous to Enter()
            StepOn(atom/movable/A)
                //always allow an object to step on itself
                if(A == src) return 1
                //do not allow two dense objects in the same turf
                else return !(density & A.density)
            //analogous to Exit()
            StepOff(atom/movable/A)
                //always allow stepping off of something by default
                return 1
            //analogous to Entered()
            SteppedOn(atom/movable/A)
            //analogous to Exited()
            SteppedOff(atom/movable/A)

turf
    Enter(atom/movable/A)
        //don't allow dense movables onto dense turfs
        if((density | loc.density) & A.density) return 0
        else
            //check contents of this turf for something else that will block A
            for(var/atom/movable/M in src)
                if(M != A)
                    if(!M.StepOn(A))
                        return 0

            return 1
    Exit(atom/movable/A)
        //check for something that will prevent A exiting src
        for(var/atom/movable/M in src)
            if(M != A)
                if(!M.StepOff(A))
                    return 0

        return 1

    Entered(atom/movable/A)
        //perform the default action
        ..()
        //call SteppedOn() for everything in contents
        for(var/atom/movable/M in src)
            if(M != A)
                M.SteppedOn(A)

    Exited(atom/movable/A)
        ..()
        for(var/atom/movable/M in src)
            if(M != A)
                M.SteppedOff(A)

atom/movable/StepOn(var/mob/M)
  if(ismob(M) && !M.CanSee(src))
    return 1
  else
    return ..()