I have a bunch of different ideas for how to do it.
1) A Lich has completed a spell. All zombies gain a strength/speed bonus. (This is an event that would be weighted to happen the longer a game goes on)
2) A vampire sabatuer was found inside one of your cities, lose 10% population in that city.
3) Dark artifacts were found in the home of your high priest. Spend 15 Mana to destroy them or sacrifice 10 population.
Stuff like that. I also want positive events too, tho.