A situation came up (thankfully avoidable), but it brought up the following question.

If a hero places a Guard Order, and a monster with Fly which is behind rubble that blocks the hero's line of sight to it, and then flies over the rubble, at which point can the hero interrupt the monster's movement to attack? Can he attack the flying creature above the rubble (targeting the square of the rubble) or does the rubble block line of sight even to the square it is in?

As written, I believe that the attack would not have LOS. Fly and Soar combined only mean that the player needs +4 Range, I'm not sure it ignores LOS rules also. As I understand it then, the player would need the extra range and LOS to the square the monster was in. Since rubble blocks LOS at the edges of the square, LOS could never be drawn to the middle of it.

I believe the player is still in advantage though because the monster should no be able to make attacks or finish their move while in a square in rubble so an overlord can't move onto the rubble, make an attack, and move away. He'd need to move on to the rubble, move to an open square with LOS, (player should guard), make his attack, and move away.

I know a lot of cards that allow movement through rubble or when it talks about sharing a square with an obstacle the only action available is usually movement out of the square. This may be covered in the moving through allies portion of the rulebook also.

I didn't think of this until later, but while in the square with rubble the monster wouldn't have LOS out of the square by the game's rules also.

Monsters, or heroes with acrobat, can not stop their movement in non-empty spaces and can, thererfore, not attack from a non-empty space.