In the "Lost Knowledge" adventure there is a Light/Dark side track which is used to defeat a Sith illusion. This reminded me about the similar tracks in WH3 for the social encounters (during audience for example). Now, in my adventure I want to use similar track where I, as a GM, use Despairs to trigger a bigger event. Example:

The characters get inside the rebel base and are arrested. They are interrogated and try to talk out from the mess. Later they need to convince one person working for the rebels to share knowledge with them. In short, they will have either to convince them that they mean no harm or use deception to get what they need or whatever other idea. There will be a lot of possibilities for a Despair (opposite checks). I thought that I will create a track (starting at 0 up to the number of PCs) where I place a token each time a character rolls a Despair (and I spend it for the track). Each position on the track means that the Empire appears near the base and start scouting the area with the last position on the track meaning that the Empire finds the base. This of course puts suspicion on the characters....

The reason for this is that I think such bigger obstacle should not be triggered with only one Despair.

What do you think of such idea? Has anyone did something similar or has other concept?

I'd be inclined to go with 4 Threat and/or 1 Despair. Despair are more likely to occur on Opposed checks, but if you limit yourself to only Despair triggering the track it may not work out the way you intend. Generally, I like your idea.

The Jewel of Yavin has something a lot like this. Without giving too much away, the PC's will be investigating some locations, tech, and people while preparing for a heist. Spent threat is usually a little more immediate, perhaps involving harassment by local security forces and such. However, if they roll a Despair, the GM can invest a Dark Side Destiny point. When the number invested equals the number of PCs, they attract some very unwanted attention. It doesn't derail the whole scenario, but it definitely introduces a big "Ohhhhhhh ffuuuuuuuuddddge."

The best part? The players don't know what's happening. They roll a Despair, and think, "Uh oh. That's not good." Then the GM reaches over, flips a Destiny point, and makes a little mark in his notes. When the PC's say, "Oh crap, what happens now?" you can just say, "Hm? Oh, nothing. It probably won't even come up. So now what do you do?"

Just be sure that the results of the Despair have some logical bearing on the roll being made. If you're slicing into a corporate computer on top of a tower, all that Despair probably shouldn't lead to a pack of Rancors attacking the city. Actually, strike that. Find a logical reason why it would lead to a Rancor attack, because that sounds pretty awesome.

I really like this idea, as I'm always looking for new ways to put FFG's awesome dice to good use... And it gives the players the idea that they're pushing their luck and flirting with disaster.

I would like to use this mechanic in an interrogation, too. However, there is only one PC that sits in the interrogation room while the other PCs try to find a way to free him. Any advices?

The captured character is making Discipline checks opposed by the interrogator's Coercion. Basically, for every check that the players make to try to rescue him, or every say, two rounds of combat, the captured player has to make another test. Failure means he gives away some minor secret. Despair however, is counted until there have been Despairs equal to the number of players, at which point he cracks and spills everything.

The captured character is making Discipline checks opposed by the interrogator's Coercion. Basically, for every check that the players make to try to rescue him, or every say, two rounds of combat, the captured player has to make another test. Failure means he gives away some minor secret. Despair however, is counted until there have been Despairs equal to the number of players, at which point he cracks and spills everything.

What about do you think about of inflicting Strain until he reaches 0 and then cracks?

That would work as well. Scathing Tirade is very much like an interrogation. If interrogator succeeds but with a lot of threat, you might let the player take the damage as wounds instead of strain. That keeps them farther from cracking and reflects the "I'll die before I tell you anything" mentality. It also lets the player feel like he has some control over the situation.

This is a random tangent, but if the other players are springing him in a stealthy manner, one thing to note is that extended stealth scenes don't work horribly well if you treat failure as being seen. If you reserve being caught in the open for failure + despair or failure + a lot of threat, then it will flow better. Failure without extreme additional side effects simply means that the party couldn't find a gap in the patrols, or two baddies started up a conversation really close by so that they couldn't keep moving. A couple threat (or maybe three, if you're feeling nice) on a successful or failed test can mean that someone thought they heard something and will go investigate. Cue another check where failure does mean being seen, but if the player can manage a good roll, the infiltration can continue.