This adventure was originally created to bridge the retirement of some rather munchkinish high-level characters with a starting party. The players had been playing good characters for a long time, and wanted to try evil. This is the result, and IMHO it turned out quite well. If you need an intro for a new party with evil characters this may work:
A group of high-level good characters arrives in a town known for its seedy- side. In short order, they end up wiping out the Thieves' Guild, Assassins' Guild, and all the evil temples and magic schools there. They also replace the former puppet government.
As this is going on, the new characters (who were members of those guilds, temples and schools) escape. (It is possible that some of the characters are friends or relatives tagging along - this allows for some non-evil characters in the group.) This is where the adventure begins - with them "on the run" from overwhelming forces. They join up after fleeing from the city. Where do they go? They aren't prepared for overland adventures or the dangers beyond the city walls. Yet returning to the city means possible capture by the new government. Rewards are being offered, so former contacts might very well turn them in.
The party may resort to stealing some horses and equipment. After a while, some bounty hunters will pick up their trail. It is easy to turn some of these bounty hunters into recurring characters that almost catch them between various adventures. (In the original campaign, one known as The Inspector was always hot on their trail. The players never realized he was just an illusionist using his spells to appear as a large garrison tracking them down.) With prices on their heads, the party will need to be extra cautious when dealing with shopkeepers who will eagerly turn them in for a quick profit. All those things that good characters normally take for granted now pose huge risks for this group.
The surrounding territory is heavily populated by humanoids and other beasts which makes escape from the city quite difficult. Just because they may have a common enemy doesn't make these creatures any less likely to view the fleeing party as food. If they do make deals with the monsters, word of this may get back to the city further ruining their reputations. As the characters become more powerful, their names and faces will appear on wanted posters in the towns they visit. Other adventurers may also try to track them down to make a name for themselves. Rumors of the party's evil-doings can get wildly exaggerated to the point that orders for their capture keep coming from higher places of office and further away.
>From a DM stand-point, it is easy to convert and use any common adventure type with this ongoing plot. For example, instead of the typical dungeon-delve, the party explores and secures a new hideout. Instead of guarding the caravan, they attempt to steal goods from it undetected. Many DM's will likely have a blast converting parts of their old adventures for this one...