[LDiCesare]

The options you have to stealthily decrease enemy dominion are:

Move a stealthy pretender or prophet there.
Stealthily preach.
Send in stealthy heretics (I'm not sure there are many - EA Arco has one, LA Pythium has many heretics but I don't think they are stealthy), or a someone with a stone idol (same effect: decrease any dominion, including yours).
There's no blowing up temple that I know of.
Hoping a tartarian besieging a castle goes appropriately mad.

You can spread a misfortune or turmoil dominion of your own hoping a bad event destroys enemy temples, but that's not more reliable than a tartarian.

For unrest, a few Bogarus units are great at raising it without doing anything special.