Vessel Fragment

Vessel Fragments are Items in Hollow Knight.

Use
Main article: Spells and Abilities

Vessel Fragments are parts of Soul Vessels. 3 Vessel Fragments complete a Soul Vessel and increases the Knight's maximum SOUL by 33. When the main Soul meter has empty space, any SOUL stored in the Vessels will automatically fill up the main meter after a short delay.

The Nail acquires SOUL for the Soul Vessels at a reduced rate, only providing 6 SOUL.
 * Soul Catcher increases this to 8 SOUL.
 * Soul Eater increases this to 12 SOUL.
 * These effects stack, giving 14 SOUL if both are equipped.

All other sources of SOUL do not fill Soul Vessels at a reduced rate, instead filling them at their normal rate.

With a total of 9 Vessel Fragments in the game, 3 Soul Vessels can be acquired, doubling the maximum amount of SOUL that can be carried (99 + (33 x 3)).

How to Acquire
There are a total of 9 Vessel Fragments to be found in the game:

Trivia

 * While the fountain in the Ancient Basin will always ask "Drop all Geo into the fountain?", only a total of will be added. Any extra will not be subtracted.
 * There are unused sprites found in the game files which suggest that a Soul Vessel originally consisted of only two Vessel Fragments, showing two halves which needed to be collected to acquire a full Soul Vessel. The "A Weird and Wonderful World" trailer also shows Sly selling one half of a Soul Vessel. This was changed to three fragments for the final release.
 * This might be why Sly still refers to his two Vessel Fragments as being a complete set.
 * The music heard after acquiring three Vessel Fragments originates from a short sample track that was posted when Christopher Larkin was hired as the game's composer.

References

Фрагмент сосуда Fragmento_de_vasija Fragment Naczynia