It seems I have once again set this blog to update at the wrong time. Sorry for that.
RPGs are funny things. Especially the campaigns that you run at your own table. Why? Well, I don't know if you've noticed, but I'd wager that the impossible happens at your table a lot more often than you realize. No, I'm not talking about the elves, dwarves, and other magical creatures. I'm talking about the simple scope and epicness of your individual campaigns. However, what do you do when your PCs are trying to do something that should be blatantly impossible?
The inspiration for this post comes from a game that I'm in. It's the Dark Heresy one for anyone interested, and basically the PCs have got it into their heads that to save humanity they need to turn off the Golden Throne. To do that, they need to amass a force that is capable of actually invading earth, get past all the legions there to defend the throne, and then figure out how to turn off a contraption that has been keeping a divine being alive for 10,000 years via the sacrifice of thousands of lives each week.
Basically, heresy of the highest order, and if you know the 40k universe, blatantly impossible.
The first thing I think a GM should do for this is identify why the events in question are impossible. Now the easy answer to this is that in your game nothing is impossible, which then leaves you with identifying why is this highly improbable? To answer that, with the example situation, you just need to look at the forces arranged against the PCs and the respective levels of those forces.
In this case, the PCs have just entered into the Ascension power level, so while they are very good they're not amongst the biggest fishes in the universe yet. Then there is the simple question of the scale of the task, the sheer number of forces that can be brought against them, and the types of forces that those will be. For example, space marines. None of the PCs are space marines, and chapters of space marines tend to make short work of any rebellion that they encounter. Also, the PCs are going to be running afoul of multiples of these being as their plan is to invade the heart of human space.
Explain The Situation
The next thing that I think the GM should do is explain the situation to the players. Let them know that they're trying to do the impossible. Do not tell them that it can't be done, just that it is going to be hard. Discuss the issue, and tell them some of the things that you've identified are going to be key problem areas. You don't want to give them the solutions (i.e. the GM shouldn't say "Space Marines are going to roflstomp you.") but you do want to present the core fact ("there is a lot of very experienced military between you and your goal. Probably an insurmountable amount.")
If the PCs still want to do it, then that is fine. Just make sure they know going in that they've chosen to play the game on Very Hard mode. The kind where you can expect a party wipe almost any time there is a combat going on.
Do It Anyhow
You are then left in the very hard GMing position of doing it anyhow. If the PCs can't be deterred, and the players know the risk going in, then you simply go with it. The players are going to try to pull off every ballsy trick they can think of in order to accomplish their goal. The problem is that you need to allow some of that to happen, present the full scale of the challenge, and still allow a chance for them to win.
This is the GM equivalent of walking a razor wire. Make it too easy and it feels cheap and handed over. Make it too hard and it gets overly frustrating and like the GM is trying to enforce their will on the outcome of events. Nail it properly though, and the PCs will feel like they've fought a good fight. Even if they don't win out their final goal, you can do it in such a way that they still have a spectacular end that leaves everyone satisfied with the outcome. And, in the end, isn't that the goal of the gaming group anyhow?