SOUL

SOUL is the term used for the white energy extracted from enemies and Soul Totems. It is represented in the player's HUD by a circular meter that fills up with white liquid, revealing two eye holes and giving the meter the appearance of a mask.

Note: All percentages in this article are estimates; there is currently no way of viewing exact SOUL values. Percentages are also rounded for simplification (33% is actually 33.333...%, etc, and 99.999% = 100%).

Using SOUL
SOUL is used to cast damage-dealing Spells or use Focus, which heals the player over time. When the player gains enough SOUL to cast a spell, the meter will glow briefly and a sound will play.

Initially, casting Spells or using Focus requires 33% of the player's SOUL meter. Using the Spell Twister charm reduces the cost to 22%.

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

If the player 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. A Soul Vessel requires 66% meter to fill completely.

If the main SOUL meter has empty space, and the player is not casting a Spell or using Focus, Soul Vessels will recharge the SOUL meter at a rate of 33% per Vessel.

It is important to note that this means it takes more SOUL to fill Soul Vessels than the Vessels return. Because of this, if a player 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, as there is a delay between running out of SOUL in the main meter and having it be charged by the Soul Vessels.

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

Attacking Enemies
Enemies struck with the Nail cause the player 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 15%
 * Soul Eater increases the amount of SOUL gained to 21%
 * The two above charms stack with a diminishing return, increasing the amount of SOUL gained to 24%

Dream Nail
Striking enemies with the Dream Nail will fill the player's SOUL meter by 33%. 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 player approaches. When hit with the Nail, they release orbs of energy that are absorbed by the player and regenerate 11% of a Vessel's worth of 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 player sits at a Bench.

There are three different sizes of Soul Totems, each granting a different amount of SOUL:
 * Small: 33%
 * Medium: 100%
 * Large: 166%

Soul Jars
Soul Jars are found in the Soul Sanctum in the City of Tears. They contain the same orbs of energy as Soul Totems, though breaking one only grants 22% meter.

Other Sources

 * Standing in a Hot Spring will slowly regenerate SOUL over time at a rate of 50% per second
 * The Grubsong charm regenerates 22% SOUL every time the player takes damage
 * The Kingsoul charm slowly regenerates SOUL over time at a rate of 2% per second
 * Salubra's Blessing will regenerate SOUL every time the player sits at a Bench, at the same rate as standing in a Hot Spring.

Trivia

 * Given the architectural style and coloring, Soul Totems seem to originate from the civilization that existed before Hallownest, similar to Arcane Eggs.
 * It's shown in the records found in the Soul Sanctum that the Soul Master and his followers were attempting to use SOUL to counteract The Infection, but the idea was rejected by The Pale King, and the researchers became overwhelmed by the power of excess SOUL.
 * It is implied that the SOUL these researchers were experimenting with was harvested from dead bugs.
 * Mistakes and Follies are presumably the result of weaker bugs becoming overwhelmed by SOUL, causing it to mutate their bodies.
 * It appears that, in contrast to the pure SOUL the player uses in Spells, the Soul energy used by the inhabitants of the Soul Sanctum is tainted with Infection due to its orange tint.