Question about fairness to players(spoilers included)

[DM Mathpro]

So I'm running a play by post game where the players start in book four. They know that sandpoint is under the threat of giant attack and are headed there to hopefully intercede before its to late. I've been thinking about having the group get there either just as the attack is getting underway or mid battle. Should I warn the players that this is going to happen(thus spoiling the surprise) or is this something I could spring on them with out dire(people quitting the game) consequences? This would be a huge twist and I could see some players feeling...trapped by the outcome...but I don't know if I would want this spoiled for me either with a pre-warning.

[GM Lamplighter]

Why would you want them to fail right off the hop? If there has been no chance to figure out what is going on, you're making them lose Sandpoint as part of your story that they can't influence. Not a great start to a campaign.

[Misroi]

Agreed - the whole point of the attack is that, had the PCs not returned to Sandpoint to bolster the defense, it would have been a LOT worse. I could see having the PCs run across a small forward scouting party of hill giants or something, but starting the battle midway? Nah, that's just not fun.

[Latrecis]

I'm afraid I'm only going to echo other responses: the pc's should know about the looming attack on Sandpoint.

At the end of Book 3, the pc's learn Sandpoint is going to be attacked and race back to town. "Race back" being an eye of the beholder thing - some groups might teleport, others might travel overland. The AP provides NO explicit timeline - this has thrown other DM's for a loop as well based on the occasional post here - the assumption is that the giant attack takes place the morning after the pc's arrive back in Sandpoint no matter how long it takes for them to get there. Barring an specific reason not to, the players should be told a giant attack is imminent and be given some time to prepare for it.

What is the background for these pc's? Is it assumed they've already completed Books 1-3? What do the pc's know about Mokmurian, Runelords, etc.?

I don't know anything about "play-by-post" so I'm not sure what impact this might have but by the time Book 4 rolls around there's not a lot of role-playing left. The rest of the AP could be summarized with the old Marine recruiting joke: travel to strange, exotic locations, meet new and interesting people and kill them. And take their stuff. (Not a dig at the AP at all, my group is starting Book 5 and it's been a good time, just an observation that there isn't many negotiation or diplomacy opportunities left. Though some groups could try that in Runeforge...)