Diablo II/Sisters to the Slaughter

The final quest of Act 1 is triggered by Deckard Cain after you complete all previous Act 1 quests. You must now make your way to the monastery catacombs and face down Andariel herself. Go back to the armory and work your way through the jail to the inner cloister. Take care, as you're bound to run into unique or champion monsters along the way, some of which may freeze you.

Pitspawn Fouldog
Super Unique Tainted (Cursed Cold Enchanted)

Inner Cloister
Waypoint

Cathedral
From the inner cloister, go to the cathedral, from which you can acess the catacombs, a 4-level dungeon where Andariel waits at the very end. It's a long, tough journey, so bring plenty of health and mana potions, as well as some town portal scrolls.

Bone Ash
Super Unique Skeleton Mage (Magic Resistant Extra Strong Cold Enchanted, so Immune to Cold even in Normal), poisonous

Catacombs Level 2
Waypoint

Andariel
Evil

Die, maggot!

Fear me!

When you make it to Andariel, you'll have plenty of minions to contend with, and Andariel can use poisonous attacks. Just keep healthy, and use town portals if you need to. So long as you keep persistently chipping away at Andariel, she'll go down, dropping some high-quality treasure. Upon returning to the rogue encampment, you can talk to Warriv to move on to Lut Gholein, the central hub for act 2.