Any ideas on how to get a 'sudden attack effect' in a board game?
It's hard to answer this without understanding the core mechanisms of the game you're talking about.

However for reference maybe you could try playing Star Wars: Rebellion.

In that game you have the Empire vs the Rebels and the Rebels pick a planet to be the rebel base.  The base is one of the planets on the map but also has a separate space on the board to represent units stationed at the secret location.

The game is card driven, each turn you play missions from cards, as well as move / attack with units.  Some of those cards are things like launching surprise attacks which come in 2 forms:


Units from the rebel base can appear on a planet suddenly, if the Empire is present they resolve a combat.
Units can be deployed from reserves on a chosen planet and resolve a combat if the Empire is present.


This system works in a game which is card driven, or has some kind of ability system as well as one where units can be deployed to the battlefield during a mission.

That being said you could adapt something similar by having units teleport to a location as a surprise attack, the rationale could be something like their armies had bad intelligence on your army, perhaps you could have it so only a small number of units could ambush at once.

You would need to decide if your game was one of perfect information, ie the current state of play is known to all players at once, I'm sure you could come up with some clever form of trap in a system of perfect information but its going to be a lot harder than something like this.