If I Knew You Were Coming I’d Have Baked a Cake!
Villages and cities are being razed over night with no warning. The few survivors speak of an entire army appearing from nowhere, blind-siding the city’s defenders.
The army in question could be traveling through the underdark, emerging from caves near their intended targets.
A great enchantment creates a field of invisibility, preventing anyone from outside the field seeing the army within it.
Or perhaps the army marches through a parallel plane, opens portals in the middle of their intended target, and overwhelms them from within.
I once used a trick like this in one of my campaigns. The Pc and their army bottlenecked their enemies using tactics and terrain to get an advantage but when they encountered the enemy, they used powerful magics to summon reinforcement from the capital a thousand miles away. Lets say that this crushed the pc’s army and drove them back into hiding for a few years before they could muster enought millitary might to be able to try it again. It was great fun for all of us.
Oh boy! This sounds like a nice plan! I might be using this soon……