How to deal with powerful stealth army?

[Saulot]

PvK said:
Which era of Ulm are you?

(PD 10+ can catch an army though, especially because there is a modifier for the size of the sneaking army.)

Not likely since that only counts units that have less than 10 stealth, which the vans don't. So the vans should be plenty difficult to catch with patrollers.

And PD below 25 (what he's likely to have in most provinces) is highly unlikely to catch the vans since it would require getting a high roll on the patrol (2d25), with at least one reroll, while the stealth roll was a total bust.

If you can't catch the raiders without just a lucky positioning of your army, and you're not yet ready to throw artillery magic at it, I'd recommend ignoring the raiders, and fight fire with fire, if he's focusing so much of his fighting resources on raiding, go send your army into his lands, and take provinces faster than he does. Invading Van is usually unpleasant because you never know when you'll bump into a large army defending one of his provinces due to the glamour, and not being properly scripted to deal with it, but since you know exactly where his army is, you also know where it isn't.

Meanwhile, focus on building more forts where you don't expect an attack during the construction.