We ran into an interesting question in this scenario. The event tile gives two options. One of them is titled, "At Your Command: The Overlord can invert two tiles in the river."
We had some difficulty interpreting the word, "Invert". It would seem to literally suggest that you could flip a couple of tiles in the river over to prevent the players from getting the XP for killing the tile, however, it's not clear what you would do with models that are left on the board when you invert the tile. I couldn't find this effect defined anywhere in the rules. Also, it would seem to be more efficient to simply reinforce those tiles using the other option (which gives you 3 RP's).
That got us to thinking that it may be an awkward translation from "switch the position of two tiles in the river". That would seem to be a more beneficial effect and less ambiguous to implement.
Can we get a ruling from Monolith? How do you guys play it?