(1) I can't remember all the details now. I know that it actually started with an Arcana check, which I hadn't anticipated - the PCs had just stopped a demonic ritual in a temple on top of a mountain, and were fleeing the temple as it collapsed behind them. The wizard PC used Arcana to stop the arcane energey spreading any further out of the temple and collapsing the mountain-side, because he didn't want the pursuing gnolls to be alerted by such a dramatic event.This sounds interesting, but I have some questions on it:
1) When were the 4 primary skills rolled and by whom? At set locations? By the best PC with the skill?
2) What happened if 3 failures occurred before 6 successes.
I think there was probably a group Endurance check in there somewhere - the rationale may have been that no one used Athletics to facilitate the climb down the mountain. I can't remember if Stealth was used.
In between checks, I was describing events for the players - the descent down the mountain, the travel to the river. I had predetermined that after a certain number of success they would find themselves in the river valley, with a heavy fog, and stumble into the gnoll funerary grounds, where they would have to deal with the gnoll shaman and followers. Because there had already been a failure at this point (can't remember what it was), the manticore-mounted gnoll joined this fight with the benefit of surprise.
After the fight, which was a fairly tough one, I remember multiple checks were made to cross the river - again, I think this may have a group check, with overall success (3 out of 5 PCs) meaning that they all made it across, but some sort of minor penalty (maybe a HS, or a piece of equipment lost - I can't remember now) for each individual failure.
After that the ranger PC rolled Nature to try to find a suitable sleeping place, and failed. The wizard then rolled Nature and succeeded - but (given that I had decided that failure meant no extended rest) I described the result as being one of finding a clearing where they could tie there horses, but that the night was a restless one, with noises, swamp insects, rain etc meaning that they failed to get the benefit of an extended rest.
Another success took them to the next predetermined encounter - a ruined manorhouse occupied by witches - and at this point things branched off in a different direction, and several sessions later the final check of the travel challenge hasn't been resolved yet.
To try and answer your question more succinctly - in this sort of SC, I am fairly relaxed about who makes what checks - which means that the players tend to organise that (for example) Nature checks are made only by those with good Nature bonuses. But once a check has been made in a given situation and failed, I don't generally let the same PC make another check. So once the ranger has failed to find somewhere to sleep, the onus falls onto another party member.
Also, as the above description indicates, when the course of action that the party describes means the whole group is involved - "we're all climbing down the mountain path" or "we're crossing the river using techniques XYZ" - then I will generally require a DMG2-style group check.
And the other thing that I think is important is to narrate the consequences of each successful or failed check in such a way as to set up the next complication. I also fit the narration to the checks - so if the players had chosen Stealth rather than Nature after they'd crossed the river, then rather than empahsising the difficulty in finding a place to rest, I would have linked the outcome to finding a hiding place - "you eventually find somewhere concealed - but it's too cramped/swampy etc for you to get a good rest overnight".
The reason I do this sort of travel via a skill challenge rather than more freeform is that it sets some parameters around the number of checks required, as one aspect of that reduces check-mongering by either GM or players, and also gives me a simple structure in which to locate the consequences of failure.
(2) A final failure means that, when they get to where they are going, they are so exhausted that they are surprised by the attacking humanoids (the encounter I have in mind combines two encounters - "Fire on the Water" and "Village Showdown" - from the 2008 Dungeon module "Heathen").
I agree that there have to be consequences. As to whether or not this includes not making it, I think that depends. In the case of the challenge I've described above, the PCs are following a river on their left with foothills on their right. So they can't get lost. And I'm not interested in a situation in which they find terrain - a cliff or waterfall or something - that they can't successfully cross.There must be consequences for failure.
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To me, it means that if you're doing an overland journey SC, there has to be a real chance that you might not ever make it.
I think exhaustion upon arrival such that the next encounter is more challenging can be sufficient. The skill challenge has produced a story that the players are engaged in - of a difficult rather than an easy trip - and the players can see from the narration in response to their skill checks how their successes and failures led to this result.
In some skill challenges the consequences of failure could be more drastic, I agree, but I tend to think of travel SCs as means to an end rather than ends in themselves.