[Executor]

There are many spells you can cast at your enemy which will appear as random event and unknowen to the target.
Flames from the sky, hits 50% of the army with
Send Horror, send a horror that attack the targeted army,
Phantasmal attack, send 25 phantasmal warriors that attack the targeted amry,
Manifestation, send an Ashen Angel that attack a commander,
Earth attack, a size 6 Earth elemental attack a random commander,
Murdering Winter, similar to Flames from the Sky but weaker,
Mind Hunt, targets enemy commander, MR negated so use penetration bonusees,

There are items you can use, bane venom charm for example.
It spreads disease in the province it is located so it can affect the army located there.
Imprint Souls can make a targeted province your depending on how powerful your mage is.

And possibly the best way is to wish for Armageddon and hit every province on the map with a volcanic eruption killing population and troops.

There are many many spells and items that have terrorist effect you want, just study the manual a little.