The PC's are traveling to a major city, housing the main temple to a high standing church. On the way they are ambushed by three to six bandits or so.
They can be of whatever level or skill the DM chooses, but the players should be able to defeat them without too much trouble, but still get dinged up in the process. If the PC's take a prisoner, then they might be able to influence him to take them to their hideout--an old abandoned watch tower a few miles into the woods. They might be able to find some nice treasures. On the leader they will find a brass necklace with a strange twisted symbol, and will also find that the leader has a extremely expensive and ornate sword. The players will obviously take this, and should for the adventure to work.
When the PCs reach the city, then they learn rumors that a high standing lord, high in the church hierarchy, has had his ten year old son abducted. A detachment of royal guards sees that the players have the sword (which really belongs to the lord) and ask them (or force them) to accompany them to the church. They take him before the lord and they discover that the lord's stepson was believed to have abducted his true heir, also escaping with the sword. The bandit leader was in fact the lord's stepson. If he is still alive (escaped or the players left him tied up if they were not the slaughtering types) then they can return him (preferably alive, but acceptable if dead) for a reward. (and of course the lord will want his sword back, which is a +2 magical weapon.
If the players apprehend the stepson efficiently, the lord hires them to find his missing heir. They will have to question the stepson and other person's involved if they are still alive, and do some probing around in the lords manor (the DM can think up details, but don't make it too difficult. When they start making headway on the investigation, they are attacked in the street by black-robed clerics with magical picks 1d4+1. If these picks even touch bare skin, the character must role a save vs. magic or suffer 1d4 points a round until cured with a Curse Disease spell. If the hitpoints fall to zero then the character only falls unconscious. They loose one point of constitution an hour until it reaches zero and they die. If they are cured then they regain a point of Con a day.
If they kill the clerics, then the discover they wear necklaces identical to the one the lord's stepson wore. The necklaces are the symbol to an evil order of spell casters (mages, priest, and their henchmen) and they magically serve to protect the wearer from the touch of the disease-enchanted items of the order. They also serve to locate each other. Priests and mages above fifth level in the order, can sense each other when they wear these. Which could add some complications for the players if they wear them. If they have any mage of cleric identify the necklaces, they will learn its powers, and the identifier will be able to sense a large concentration of the order half a day's journey away from the city at an abandoned temple.
To basically sum it up, the evil order is attempting to sacrifice the lord's son to their god during a holy night that will increase their power and doom the church based in the city. Many things could happen now. The players can rush in a save the day, maybe with the help of some of the lord's troops, and gain a huge reward... Whatever you do, give the players a chance to save the heir, but also make it a huge challenge. If they fail to stop the sacrifice, it doesn't necessarily mean they are dead. When I used this adventure, the character's success depended on an initiative roll. They failed, and the heir was sacrificed. It turned out that the Second hand Priest of the church participated in the ceremony, and the evil order invaded the city through ancient hidden tunnels, and took the whole church, capital, and palace.
Basically, they took the whole city. The lord died as well and the players got to keep his sword. I took this little adventure and even turned it into a huge campaign, where the forces of the church had to flee from the capital, and seek help from other nations to regain the city. The evil order was also in league with a nation of orc's in the north that had been warring with the nation for years. The order would weaken the nation, while the orcs rushed into attack. In exchange, the orcs would give the leader of the evil order a powerful artifact that they possessed, which could cause major problems for the players latter on...
Of course, the DM can take this wherever he wants, and there are plenty of options and other adventures that can spring from this. Hope you like it.