Act 1 Quest 1: Den of Evil - Diablo 2
Quest Location: Den of Evil
Quest NPC: Akara in Rogue Encampment
Quest Goal: Clear all Monsters inside
Reward: 1 Skill Point & 1 Free Respec per difficulty
The Den of Evil is the first dungeon you'll be completing in Diablo 2 and is quite straight forward. Your goal is to defeat every single enemy in the dungeon, the only way you'll typically miss an enemy is if you don't notice one of the Fallen guys being resurrected. For those of you wondering where the last enemy is in the Den of Evil, that's probably what you missed.
Inside this dungeon you'll find Corpsefire, a Super Unique enemy which always spawns here. Once you've killed every enemy inside of the Den of Evil light will shine down from the ceiling and it'll be time to return to Akara in the Rogue Encampment and speak with her to complete the quest.
Note: While this quest is not required to complete the game, it gives you 1 Skill Point which is an amazing reward. It's recommended you complete this on every single difficulty for that reward. For more information about quests like these that give permanent rewards follow the link provided.