"Use to sacrifice yourself & unlock a powerful being"

The Lightsworn Crystal is an item used to unlock the Lightsworn race. Doing so will instantly kill your character. Specifically, the Lightsworn Crystal deals 19,998 damage on use and bypasses anti-OHKO measures (normally, you cannot be killed in one hit if above 1 HP), making it useful for acquiring unlocks requiring one's character to take a lot of damage if you don't have access to Peasant Uniform yet.

It can be purchased at Kylocke's Treasures stall in the market for 50,000 World Fragments.