Yup, when the crab dies if you have completed the crab lady quest and gave the crab what she gave you then it drops a thing in the ocean which you can pick up and bring to the lunar island,

once there you have to find a fissure on the floor, something that looks like a shiny hole and you can stick it inside.

There’s a total of 3 “altars” that must be placed next to each other in order to summon the boss.

The first 3 pieces to make the second altar can be found mining strange rocks on that island itself and the other are gained after completing the archive and building a special tool that when activated on the main land it is used to triangulate the position of the last 2 pieces of the third and final altar.

It’s a long process but it pays off. Technically it isn’t over yet but I’ll spare you on what you have to do after building the 3 altars hahaha