The last quest is of course to kill Diablo. You talk to Tyrael for this. Hadriel is near the River of Flame, and tells you about the 5 seals. When you go into the Chaos Sanctuary you have to go through hordes of monsters to get to them. Also, when you trigger the seals, named monsters in groups of their 'type' of monsters appear to attack you. Have lots of town portals ready.

When you get to all 5 seals and Diablo appears, I use a hit-and-run method and can do it without dying. I used a Druid this last time through and simply had a good fissure spell that I kept dropping below his feet. My 'helper' had a cold arrow that she kept freezing him with, and between the two he went down without much of a fight. That's the end!

Diablo 2 Walkthrough
Walkthrough Master Index