In a recent combat, an Ogre mage was invisible, flying somewhere, and I wanted to ready an attack (with a magic missle spell of my own) to disrupt his casting. My ready trigger was to hit him with the MM when I saw him or heard him, with a clear intent expressed to my DM that I was trying to disrupt his casting (I was prepared that if he didn't cast, i would lose my action). Which of the following options do you think is correct?

1. He appears as he STARTS casting, allowing me to hit him with my readied action.

2. He appears as he FINISHES casting, so I have no chance of disrupting his casting.

3. He appears as he STARTS casting, but I can't get a MM spell off at him, unless it was quickened or something. Should'a used a bow.

4. He appears as he FINISHES casting, so even a bow wouldn't have helped.

5. He appears as he finishes casting, so even a bow wouldn't have helped, HOWEVER, you are allowed a heroic (30-40+ DC) listen check to hear his speaking words 50 feet up, then a semi-heroic (20-30+ DC) spot check to target his invisible "shape" (and still have 50% miss chance due to concealment). None of that's in the rules, really, it's just the DM being "nice".

As a DM, I was uncomfortable with basic (non-improved) invisibility trumping the "ready" action. Considering that the "ready" action is now almost the only way you can disrupt a caster, I was concerned about balance. However, I was a player at the time, and my DM decided to go with option #5.

I have done search for this on the board, but the only thing that comes close are discussions about readying vs stilled and quicked spells:

What do people here think?

Thelbar