Sisters to the Slaughter is the final quest of Act I in Diablo 2: Resurrected. It sees you tasked with tracking down and defeating Andariel, the Maiden on Anguish.
The quest is formally assigned by Deckard Cain so can be triggered by speaking to him. Completing this quest will unlock Act II which takes place in the deserts of Aranoch.
Sisters to the Slaughter Quest Guide
Once you have taken the quest from Deckard Cain, you want to head straight for Eastgate Keep and the Rogue Monastery. You'll need to reach the fourth level of the Catacombs which is not an easy task.
Make sure you have plenty of town scrolls just in case you need to drop back to sell gear or just need a rest to sort your skills/levelling. When you finally reach the fourth level and find Andariel, things get serious.
It's best to take care of the mob enemies that surround her first. Your best approach could be to fight her in the rooms with the blood pool in the centre as you can use it to kite her as she chases you around it. It also gives you space to use potions and a town scroll if needed.
When fighting at range, Andariel can use a large poison AOE attack which will kill your minions and mercenaries very quickly. She can still poison you when fighting up close and personal but it's a little less effective. Do your best and do not approach the fight looking to end it as quick as possible. If you are patient and manage your health and mana well, you'll be victorious in no time.
Quest Completion
Upon completing the quest, you will unlock Act 2 of Diablo 2: Resurrected. Deckard Cain will commit to assisting you on the rest of your journey and follow you through to Aranoch. His quest completion dialogue is as follows;
"This is a great victory indeed, but many more battles await. I will accompany you on your journey, lending what assistance I can... Remember...Diablo is still out there, seeking something in the desert. I'm afraid that this nightmare will not end until you find what it is that he seeks."