betrayal at house on the hill attacking and lucky stone
The rulebook is not clear on if combat is simultaneous, or if the attacker rolls before the defender. The only thing is says is:


  When you make an attack, roll a number of dice equal to your Might. Your opponent does the same. 


Normally, it makes no difference, but with the Lucky Stone; you must decide if you want combat to be simultaneous or not. There are many little rules like this in Betrayal where your group needs to decide which way to play it. Either way, the Lucky Stone can be used, but your knowledge would be different in each case.

The Lucky Stone is used "after you attempt a roll of any kind". It is reasonable to assume this means "immediately after", or before anything else has happened. Therefore, there are 2 options:

If you play with simultaneous attacks, then the player with the Lucky Stone can choose if they want to use the Lucky Stone after the players have rolled their dice. The player will know how the defender rolled before making that choice.

If you play where the attacker rolls first, and then the defender rolls, then the attack should have to decide to use the Lucky Stone after rolling, before they can know what the defender will roll.

In the second case, the Lucky Stone is still useful, but using it might be a risk as the defender could end up rolling really poorly so that they didn't end up needing to have used it.