Act 1 Quest 2: Sisters' Burial Grounds - Diablo 2
Quest Location: Burial Grounds
Quest NPC: Kashya in Rogue Encampment
Quest Goal: Defeat Blood Raven
Reward: Rogue Mercenary & Ability to hire Mercenaries from Kashya
Flag: Ability to hire Mercenaries from Kashya
For this quest you'll need to travel to the Burial Grounds which is right next to Cold Plains and defeat Blood Raven. The zone isn't that big and Blood Raven will be in the center of the graveyard surrounded by many undead. She has high resists to all magic types and throughout the fight will summon zombies to attack you which should be ignored.
Turning in the quest to Kashya will get you a free Mercenary and the ability to hire more from Kashya if you'd like to. You can still hire Mercenaries in later acts even if you don't complete this quest, but you'll never be able to hire one from Kashya until you do this quest.
Rogue (Act 1) Mercenary
In the same graveyard as Blood Raven you'll find two dungeons, Crypt and Mausoleum. Both of these dungeons are good to run on Hell difficulty if you're looking to farm up some loot. Crypt has a 100% chance to spawn a Golden Chest and Mausoleum is the first level 85 zone that you'll encounter on Hell difficulty.
Full map of The Burial Grounds. Click to enlarge.