Act IV Quest 1: The Fallen Angel - Diablo 2
Quest Location: Plains of Despair
Quest NPC: Tyrael
Quest Goal: Defeat Izual & speak with his spirit
Reward: 2 Skill Points
This quest begins the moment you portal into The Pandemonium Fortress and begin Act IV. All you need to do for this quest is find Izual in the Plains of Despair and defeat him. Once defeated a spirit will appear that you'll have to speak to in order to update your quest.
After speaking with the spirit return to The Pandemonium Fortress and speak with Tyrael to complete the quest. That's all there is to it! While this quest is not required to complete the game, it gives you 2 Skill Points which is an amazing reward in and of itself. It's recommended you complete this on every single difficulty solely for that reward. For more information about quests like these that give permanent rewards follow the link provided.
Note: Sometimes this quest will 'bug out' if someone in your group turns it in while you're not in town and it won't grant you the skill points. If this happens to you, leave the game and rejoin it and the quest should no longer show as completed and should say "Speak with Tyrael for your reward'.