Carefree Melody

Usefulness
Over an infinite series of masks, will activate approximately 22.3794% of the time. However, since each activation itself returns another opportunity to advance the process, this comes out to an average 28.8317% increase in effective health, and since this health is not added through masks, it doubles as a 28.8317% increase to effective health gained per heal as well, regardless of the healing method.

Has a 0, 10, 18, 21.6, 25.2, 17.64, and 6.048% chance to have its next activation occur on the first, second, third, fourth, fifth, sixth, and seventh hits respectively, with a remaining 1.512% chance to reach the 90% range. This makes it require just over 293 hits on average to reach said range if preparing it for a boss fight. Comparatively, making it into the 70% range only requires about 46, and is enough to make it superior to even the Lifeblood Core in all cases barring those which involve Grubberfly's Elegy, as is possessing at least 14 total masks throughout a fight, or just 13 against sources of double damage.

Will allow the effects of other charms to continue through or activate alongside its own effect, save for Grubsong which requires taking actual damage.

Will operate outside of the Baldur Shell, benefiting from hits to the shell while also having a chance to protect it.

Preserves its odds after being unequipped, allowing high odds to be saved for any one section of a pantheon. However, its odds cannot be raised without a source of self-harm, preventing it from being used more than once on radiant difficulty. Nonetheless, it will provide a 100% increase to effective health in this context assuming it activates, making it comparable to offensive charms unless time is a concern.

Probability
The chance to activate when hit depends on how many hits have been taken since the charm last activated:

How to Acquire
After banishing the Grimm Troupe, the charm can be acquired from Nymm by talking to him in Dirtmouth.

Trivia

 * This charm takes up the spot that Grimmchild was in previously.
 * The Delicate Flower will be destroyed even if damage was negated by Carefree Melody.
 * Carefree Melody is inaccessible in Godseeker Mode.

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