Soul

Soul is an arcane force that is generated from bugs and is that substance that animates their bodies. When put to use it can be twisted into spells or even be used to heal damage. It is can also be stored in jar-like caches or ancient artifacts called "Soul Totems".

In game SOUL is represented on the HUD by a circular meter that fills up with white liquid, revealing two eye holes and giving the meter the appearance of a face. 99 is the maximum amount of SOUL the SOUL meter can hold.

Usage
SOUL is used to cast damage-dealing Spells or to Focus, which heals the Knight over time. When there is enough SOUL to cast a spell, the meter will flash briefly accompanied by a soft sound effect.

Casting Spells or using Focus requires 33 SOUL. Using the Spell Twister charm reduces the cost of casting Spells to 24.

Acquiring SOUL
There are several ways to acquire SOUL throughout the game:

Attacking Enemies
Enemies struck with the Nail cause the Knight to gain SOUL at a rate of 11 per hit. Several Charms can modify this amount:
 * Soul Catcher increases the amount of SOUL gained to 14
 * Soul Eater increases the amount of SOUL gained to 19
 * The two above charms stack, doubling the amount of SOUL gained to 22

Dream Nail
Striking enemies with the Dream Nail will give 33 SOUL. The Dream Wielder charm doubles this amount to 66.

Soul Totems
Soul Totems are structures found throughout Hallownest. They glow with Soul energy when the Knight approaches. When hit with the Nail, they release orbs of energy that are absorbed by the Soul Vessels and generate 16 or 18 SOUL.

After a certain amount of hits, the statue will cease to glow and will no longer give SOUL. It will be recharged when the Knight sits at a Bench.

There are two standard sizes of Soul Totems, though there are several different appearances for each one: Additionally, there are several unique Soul Totems with unlimited SOUL scattered throughout the Path of Pain within the White Palace. These totems are in the depiction of Pure Vessel.
 * Small (3 hits): 50 to 54 SOUL
 * Large (5 hits): 82 to 86 SOUL

Soul Caches
Soul Caches are found in the Soul Sanctum in the City of Tears. They are glass jars containing the same orbs of energy as Soul Totems, though they can only be hit once before shattering. Breaking one grants 20 or 22 SOUL. Two more Soul Caches can be found within The Abyss in the lighthouse.

Other Sources

 * Standing in a Hot Spring will generate 50 SOUL per second.
 * The Grubsong charm regenerates 15 SOUL every time the Knight takes damage, or 25 if Grubberfly's Elegy is equipped.
 * The Weaversong charm combined with Grubsong gives 3 SOUL whenever the Weaverlings hit an enemy.
 * The Kingsoul charm passively generates 4 SOUL every 2 seconds.
 * Salubra's Blessing generates 50 SOUL per second while the Knight sits at a Bench.

Soul Vessels
Soul Vessels are additional storage for SOUL, represented as small circles to the left of the main SOUL meter. The Knight can acquire 3 total throughout the game by collecting Vessel Fragments. 3 Vessel Fragments are required to make one Soul Vessel. Each Soul Vessel can contain 33 SOUL.

If the Knight gains SOUL while their main SOUL meter is full, the SOUL will instead be transferred to one of the Soul Vessels, starting with the lowest one.

Soul Vessels will transfer their SOUL into the main vessel when it has empty space and when SOUL is not being used. There is also a small delay between running out of SOUL in the main meter and having it be refilled by the Soul Vessels.

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 of filling them at their normal rate.

It takes more Nail hits to fill Soul Vessels than the Vessels return in SOUL. Because of this, if the Knight is in a long fight and is constantly gaining SOUL to use for Spells or Focus, it is better to use the main meter as soon as possible.

Trivia

 * Given what Relic Seeker Lemm says about them, Soul Totems originate from the civilisation that existed before Hallownest, along with the Arcane Eggs.
 * It is shown in the Lore Tablets found in the Soul Sanctum that the Soul Master and his followers were attempting to use SOUL to counteract the Infection and to somehow achieve immortality. This idea was opposed by the Pale King, and the researchers became overwhelmed by the power of excess SOUL.
 * The SOUL these scholars were experimenting with was harvested from dead bugs. As noted by a Lore Tablet and shown by the mass of corpses in the northwest corner of the Soul Sanctum.
 * Mistakes and Follies are presumably the result of some of the scholars becoming overwhelmed by SOUL, causing it to mutate their bodies.