In the world of Aeril, a soul gem can be used to capture the Essence of a slain creature, which can then be used to create a magic item, enhance an existing magic item, refill an item's spent charges, or even temporarily enchant a mundane item.

Soul gems are categorized into classes, which denote the challenge rating of creatures whose Essence they can capture. Gems below Class 1 are called Charge Gems, as they can only be used to refill a magic item's spent charges - not to create new enchantments. Soul gems increase in rarity at higher class ratings.

Charge Gems: CR 0-1/2 (Common)
Class 1: CR 1-4 (Uncommon)
Class 2: CR 5-8 (Rare)
Class 3: CR 9-12 (Very Rare)
Class 4: CR 13-16 (Very Rare)
Class 5: CR 17-20 (Legendary)

Soul gems of Class 6 and higher are theoretically possible, but none have ever been found.

Capturing Essence. To capture a creature's Essence, you must touch a soul gem of the appropriate class rating or higher to its body within 1 minute after it has died. A gentle repose spell works to extend this time limit. Capturing a creature's Essence takes 3 rounds, during which the capturer must use their action on each of their turns to complete the process.

Refilling Charges. To refill a magic item's spent charges using a soul gem, you must use an action to touch the gem to the magic item. The item can then regain a number of charges up to the soul gem's class rating x 3 (charge gems restore 1 charge).

Creating Magic Items. When used to enchant weapons and armor with simple numerical bonuses, the soul gem's class rating determines the bonus that is applied to that item (e.g. a Class 1 soul gem grants an item a +1 enchantment, and so on). The maximum bonus that can be applied to a weapon or armor is +3.

In order to enchant a full set of armor, a soul gem of equal or greater class rating than the desired bonus must be used for each part of the armor - arms, legs, torso, and head (if applicable) - for a total of at least 5 gems. Soul gems large enough to capture the Essence of multiple creatures can be used for multiple armor parts. The resulting bonus will be equal to the lowest class rated soul gem used.

For enchantments which add an additional damage type to a weapon (such as fire, cold, lightning, etc.), the enchanted weapon gains the ability to deal bonus damage of the desired type equal to 1d6 x the class rating of the soul gem used to enchant it. The effect is powered by a number of charges equal to the creator's proficiency bonus (at the time of enchantment) + 1 and must be activated as a bonus action, lasting for one minute or until it is dispelled. The weapon regains its expended charges at dawn.

To create an item which can be used to cast a spell, the creator must either know the spell or be assisted by someone who does. The maximum level of spell which can be given to the item is equal to the soul gem's class rating. The item is powered by a number of charges equal to the creator's proficiency bonus (at the time of enchantment) + 1, regaining expended charges at dawn.

In order for an item to bear a permanent enchantment, the soul gem(s) must be ground into a fine dust and worked into the item during its creation. Any pre-existing item enchanted with a soul gem loses its enchantment after 10 days.

Once a soul gem has been filled and emptied, it cannot be filled again.