Beat Firmament (final level) for the second time with the handmade necklace equipped. To get the Handmade Necklace collect all 7 of the stage gems and talk to Jule.

Upon cashing in 3000+ or more Kudos via a checkpoint or offensive skill, a special pendant will become available to be synthed. When equipped, it will cause a specific Anthem to be played instead of the usual track. Here is a list of where each song can be earned:

Song: Beyond the Blue, Stage: 1st Sumeragi
Song: Nebulous Clock, Stage: Subaquatic Base
Song: Azure Door, Stage: Media Tower
Song: Pain from the Past, Stage: Biochem Plant
Song: Scorching Journey, Stage: Datastore
Song: Rouge Shimmer, Stage: Stratacombs
Song: A Zip To the Moon, Stage: Pharma Lab

After beating the true final boss, you'll unlock 5 special missions. 4 of them are gauntlet-style remixes of the normal stages, featuring many more enemies, combined layouts of certain stages, and lack checkpoints.

The 5 special mission is a Boss Rush.