The Millennial Pearl Seahorse is one of the more unique Field Bosses in Genshin Impact, found in the underwater city of Fontaine. Here's a detailed overview.
Image via HoYoverse.
The Basics
•Appearance and Lore: This boss resembles a large, serpentine seahorse enveloped in a pearl-like shell, glowing with mystical energy. Its design reflects the elegance and mystery of the ocean's depths, embodying the theme of ancient aquatic creatures in Teyvat.
•Location: Fontaine: Specifically located in an underwater area of Fontaine, players need to unlock the ability to breathe underwater or use a related artifact to reach this boss.
Combat Mechanics
Phases
•Initial Phase: The seahorse starts partially encased in its shell, which must be broken to fully engage with it.
•Elemental Shift: After breaking its shell, it might shift between Hydro and Electro attacks, requiring players to adapt their strategy.
•Regeneration: At certain health thresholds, it might attempt to regenerate or unleash a more devastating attack, which can be interrupted for additional damage.
Attacks
•Hydro Beam: Emits a powerful Hydro beam that sweeps across the battlefield.
•Electro Orbs: Summons orbs that chase players, dealing Electro damage on contact.
•Bubble Trap: Creates bubbles that can trap players, dealing continuous Hydro damage or restricting movement.
•Shockwave: Emits shockwaves in water that can push players back or disrupt their position.
Strategy 
•Elemental Weakness: While primarily Hydro, it's vulnerable to Electro reactions like Electro-Charged (Hydro + Electro) or Overload (Pyro + Electro).
Avoiding Damage
•Use characters with high mobility or underwater navigation skills like Hu Tao (with her passive) or characters with good control over their movement like Kokomi.
•Characters with shields, especially those like Zhongli, can mitigate damage from both Hydro and Electro attacks.
DPS Focus
•Focus on breaking its shell first, then switch to high burst damage characters like Eula or Keqing when vulnerable.
Elemental Reactions
•Triggering reactions can not only deal damage but also disrupt its attacks or change the battlefield conditions.
•Healing and Shielding: Given its mix of Hydro and Electro attacks, having a versatile healer like Kokomi or a shield like Noelle can be very beneficial.
Team Composition
•DPS: Characters like Keqing for Electro or Ayato for Hydro can exploit reactions.
•Supports: Characters like Xingqiu for Hydro or Fischl for Electro to set up reactions.
•Healer: Characters like Barbara for healing with Hydro resonance or Qiqi for Cryo resonance.
•Utility: Zhongli for his shield or Sucrose for Elemental Mastery boost and crowd control.
Rewards
•Defeating the Millennial Pearl Seahorse grants:
•Primogems
•Hero's Wit
•Mystic Enhancement Ore
•Talent materials like "Teachings of Justice" or higher tiers, potentially related to Hydro or Electro.
Tips: 
•Reaction Management: Utilize elemental reactions to control the fight, especially when dealing with its elemental field mechanics.
•Interrupting: Focus high damage when it attempts to regenerate or use its "Bubble Trap" to prevent or mitigate its effects.
•Mobility: Stay mobile to avoid its area attacks or to quickly reposition when it shifts between Hydro and Electro states.
•Environmental Awareness: Use any environmental features around the battlefield for cover or strategic positioning.
The Millennial Pearl Seahorse presents a unique challenge with its aquatic environment, requiring players to adapt to underwater combat mechanics while dealing with its elemental shifts, making it a compelling addition to Genshin Impact's diverse array of Field Bosses.
