Grubberfly's Elegy

Description
''Contains the gratitude of grubs who will move to the next stage of their lives. Imbues weapons with a holy strength.''

When the bearer is at full health, they will fire beams of white-hot energy from their nail.

How to Acquire
Gifted by Grubfather after freeing all 40 Grubs.

Trivia

 * The beams of energy are likely a reference to The Legend of Zelda. In multiple installments of the series, the main character Link can fire ranged attacks from their sword, but in most games to feature this he can only do so when his health is full.
 * After all of the Grubs are freed and this charm is obtained, resting at a bench and visiting the room again will reveal that the Grubfather has eaten all of his children. The name of the charm, Grubberfly's Elegy, could be foreshadowing this, since an "elegy" is typically a poem mourning the dead.
 * However, many fans theorize that the Grubfather is actually protecting his children by serving as a cocoon for them to metamorphose. This is possibly supported by the description of the charm and the name of the achievement for rescuing the Grubs, "Metamorphosis". In that optic, the "Elegy" could be for the Grubfather, since upon completing their transformation his children will presumably burst out of him, killing him.