Arguably the best Lifeblood charm, as it provides twice as many masks as Lifeblood Heart with only one extra notch cost, and does not turn all masks into Lifeblood, unlike Joni's Blessing, allowing for healing if necessary.
However, it is the most difficult to obtain, requiring either Lifeblood masks from the Lifeblood Cocoons scattered throughout the area (which will disappear upon resting at a Bench to be collected and/or aforementioned Lifeblood charms to be used to unlock the door. The most reliable method is to acquire 7 or more health through collecting Mask Shards, and then equipping Fragile/Unbreakable Heart, Lifeblood Heart and Joni's Blessing to reach 15 Lifeblood masks. Then the door still has to be reached without taking any damage.
How to Acquire
On the left wall of the Abyss is a cave that will only open when having 15 or more Lifeblood masks, either through killing Lifeseeds or through the Lifeblood Heart and Joni's Blessing charms. Joni's Blessing also converts the extra health from Fragile Heart into Lifeblood masks. The charm is found sitting on a dais at the end of the chamber.
There is also an Arcane Egg in the same area. Once Lifeblood Core is picked up, the Knight will be immediately ejected from the room and the door will become permanently closed, so it is recommended to pick up the Arcane Egg first and then the charm.
Stacks with both Lifeblood Heart and Lifeblood Core.
Stacks with both Joni's Blessing and Lifeblood Core.
Grubberfly's Elegy only counts mask health as full health, so by using Lifeblood health, taking damage will not remove Grubberfly's Elegy's effect.