So it has been agreed upon by myself and my DM that once our current campaign finishes, he will become a player, and I will finally get to try my hand at the DM seat. This will be my first time running any game for any system, and the fact that it will be a Planescape campaign makes it more than a little daunting.
Here's the deal. Since none of the players are familiar with the setting the first few sessions will take place on the Prime, specifically Oerth. I'm going to try to engineer it so that completion of the first adventure will bring them to second level. At that time, the PCs will be in a town, and they will have made friends with an aasimar fighter/bard (the head of a sect that I wrote up called the Etchers of the Mark) that has taken a liking to them; he's the one that will have sent them on their first adventure, and they probably won't know he's an aasimar. The day after they get back from the task he sent them on they should be having breakfast at the inn they are staying at when the sounds of explosions and screaming erupt from outside. Then the door bursts open and in walks a githyanki that is dead set on slaughtering everyone in the establishment. The aasimar rushes up to the githyanki and tells the PCs to run, to get to a particular point in the center of town, and to not engage the enemy as they are far too powerful for them to take on directly. He then throws them something that he's been wearing around his neck, a thin iron chain with a ribbon woven through it, that the githyanki also happens to be wearing, as well as his belt pouch which will contain a few minor magic and mundane items.
So here's the situation, the town is infested with githyanki (20 - 30, most no higher than 6th level). The perimiter of the town has been secured before they made their move; the PCs aren't getting out that way. They need to get out of the inn (near the edge of town) to some place in the center of town, where the portal is. I need to figure out "encounters" that are mostly puzzles that can get them through the town relatively safely so long as they keep their wits and don't dawdle. I also want to give them the oportunity to rescue a few people along the way, however doing so means that they will have to secure a second key, which means taking out a Githyanki. I'm not opposed to one or two PCs dieing in this scenario. At least one is preferable, three or more is excessive.
Anyone have advice? Is this just an all around bad idea?
It seems like a good idea, for the most part. A PC being likely to die so early in the game, particularly as they are escaping from a doomed town for an indeterminate period of time, seems a bit cruel. It would either put one of the players out of commission for quite a while or force him/her to reroll. While the PC's are escaping, their choices should reflect or influence their alignments, which will be important out on the planes. The number of people they choose to save (or sacrifice to ease their way) will reflect the good/evil axis while the amount that they use the town's chaos to their advantage (stealing from abandoned shops, setting things on fire, etc) will reflect the law/chaos axis. Keep in mind that they probably won't know the chain/ribbon thingey for a portal key, nor will they know to get more of them to save people. A dead or severely wounded githyanki near the portal, perhaps with a small challenge to get to it, would help focus the encounter more on figuring out how portals work rather than on actually fighting the enemies. Ooh, and as a final note, if you've ever played Half-Life 2, you could turn the dying town into a sort of Ravenholm, where there are plenty of traps to be used against the 'yanki's by somebody canny enough to spot them.
Anyway, that's my two pence. Good luck on your first campaign. You're luckier than I in that you actually know people that will commit to one.