To clarify, shouldn't the NPC only reward those who participated in the invasion?
Here is a initial version. I do not have a test server, but I believe it is error free. Let me know if it doesn't work!
[paste=8qvazgg2e5b]
You will need to configure the area spawn under the Config section. Also you will need to place the reward NPC to your liking.
The invasion will automatically start at the times you provided. However, these times will be based on your server time. Make sure your server is set to GMT + 3 to have the event start at the correct time