Mermaid Island Conquest is something you can do with your Alliance members to earn a lot of rewards.
Your Alliance Skill [Mermaid Island Conquest] must be activated in order to start Mermaid Island Conquest. When R4 or higher ranking Alliance Member uses this Alliance Skill, a Mermaid will be summoned near your Alliance Fortress.
You can attack the summoned Mermaid individually or using Rally attacks. Just like other Raid Monsters, you will defeat the monster when its HP hits 0.
But please keep in mind that you have a time limit of 10 minutes to defeat the summoned Mermaid.
When you successfully defeat a Mermaid, you can summon a more powerful Mermaid that's up to 30 levels higher than the previous Mermaid.
There are two types of rewards: individual and for all Alliance memberes. The more powerful Mermaids you defeat and the more you contribute, the better your rewards will be.
Lastly, please remember that when you've equipped Tier 3 or higher Seasoned Captain's Masterpiece Equipment Set, the damage you receive will be reduced.