Here's a detailed guide for the "Abyss Mage" section, which is indeed part of Prologue: Act III - Song of the Dragon and Freedom in Genshin Impact. Remember, this guide reflects the general gameplay as of this writing. If there have been recent changes or updates to Genshin Impact specifics like rewards or exact quest mechanics might differ.
Unlocking the Quest 
Prerequisites
•Complete Prologue: Act II - For a Tomorrow Without Tears.
•Ensure you are at least Adventure Rank 18 before proceeding.
Quest Walkthrough:
1.Starting the Quest: After completing Act II, this quest will automatically begin when you log in or when you approach the quest marker in Mondstadt.
2.Meet with Amber, Jean, and Venti:
3.Head to the Knights of Favonius Headquarters where you'll discuss the recent events involving the Abyss Order.
4.Investigate the Abyss Mage: Follow the marker to investigate signs of the Abyss Mage. This will likely take you outside Mondstadt, perhaps to areas like Starfell Valley or Whispering Woods.
5.Battle with the Abyss Mage: You'll encounter an Abyss Mage. Here are some tips for the fight:
6.Elemental Shield: Abyss Mages are protected by elemental shields (Hydro, Pyro, Cryo). Use elements that counter their shield for quicker takedowns. For instance, use Electro or Cryo on a Hydro shield.
7.Teleportation: They can teleport around the battlefield, so be ready to chase them down or use ranged attacks.
8.Follow the Trail: After defeating the Abyss Mage, you might need to follow clues or elemental traces left by other members of the Abyss Order.
Discover Dvalin's Location
1.Your investigation leads you to realize Dvalin (Stormterror) is in danger or being manipulated. This part might involve interacting with elements or solving puzzles to reveal locations or mechanisms.
2.Confrontation or Rescue Operation: This could lead to another combat scenario or a rescue mission where you might need to free or calm Dvalin, involving combat or strategic use of Venti's abilities to navigate or manipulate the environment.
3.Return to Mondstadt: After dealing with immediate threats, head back to report to Jean and further discuss the situation with Venti.
4.Conclusion and Preparation for Next Steps: The quest concludes with discussions on how to handle the Abyss Order's plans, setting the stage for the next part of the questline which will involve more about Dvalin and the Abyss Order.
Rewards
•Adventure Experience: Significant amount for progressing through the Archon Quest.
•Primogems: For quest completion.
•Character EXP Materials: To level up your characters.
•Mora: For general use.
•Enhancement Ores: For weapon enhancement.
Quest Tips 
•Elemental Synergy: Make sure your party has good elemental coverage to deal with different Abyss Mage shields effectively.
•Healer in Party: Include a healer like Barbara or Qiqi since you might take damage during the fights, especially if this is your first encounter with an Abyss Mage.
•Exploration: Use this opportunity to explore the areas you're led to; there might be chests or resources you'll want to collect.
•Venti's Utility: If you have Venti, his ability to create updrafts can be useful for reaching certain areas or escaping from enemies.
Remember, this quest is part of the main storyline, so it progresses the narrative significantly, introducing key elements of the game's lore. If there have been updates or changes to this part of the quest since my last update, the in-game quest guide or recent online resources would be the best places to check for the most current details.
