The last quest is taking on the nasty bitch Andariel who's hiding in the bottom of the Cathedral. Getting to her is going to be your longest journey yet. Head back to the Barracks and go down into the Jail. Three levels later, you'll come up into the Inner Cloister. This will take you to the Cathedral that leads to the Catacombs. She is on the 4th level of the catacombs, make sure you hit the waypoints in Jail level 1, the Inner Cloister and Catacomb level 2. If you are at all concerned about dying, once you have cleared out the 3rd level of the catacombs, just prior to heading down into the 4th, cast a town portal but leave it there. Head to level 4 for a big fight, don't be surprised if you die, she is very nasty and uses a brutal poison attack, but if you have the town portal it's a quick trip back. Don't forget to cast another one before you go to get your body.
Once she's dead, head back to town and talk to everyone. They all love you. If you've got the urge to hit some unexplored areas, go for it. You won't be back here. But if your ready head to Wariv and tell him you are ready to go East. He'll pack the camp and away you'll go!