This guide will let you know how to find the Legacy Island in King Legacy.
The first thing you need is a legacy pose because, if you do not has the pose you have to search the whole second sea.
The legacy island/laugh tale spawns in a public server, not in any private server and it spawns after every 1 hour.
And it spawns very far so you should have a fast boat or fly devil fruit, if you have fast flying devil fruit then you can find it easier. So legacy pose will give you the direction as soon as the legacy island spawns.
Just keep the server hopping and you will find the island (it only spawns in the second sea).
Around the legacy island, there will be a sea beast as a protector, he has 7.5 mill health and it is hard to beat him. So go there with your gang of four plus.
As a reward you will get gems, beli and a random devil fruit from a chest, if you beat a sea beast.
One thing to note there are three levels of chest which gives different varieties of loot, it may depend on how many times you complete the legacy island raid.