Way of the Cobalt Soul

The Way of the Cobalt Soul is a monastic tradition practiced by members of the Cobalt Soul. This subclass was created by Matthew Mercer and first published in the Tal'Dorei Campaign Setting. [1] [1]  Afterward appeared in Campaign Two, still changing its mechanics until was published on October 16, 2020, by D&D Beyond.

Powers and abilities
During the second campaign, "Way of Cobalt Soul" constantly changed until episode, when these alterations were instituted in the campaign and published via Dropbox and Twitter shortly after.

The subclass was published to D&D Beyond on October 16, 2021 with a further change. Unless otherwise stated, the following mechanical information reflects Way of the Cobalt Soul as published through D&D Beyond.

Extract Aspects
Beginning at level 3, hitting a creature with an attack granted by Flurry of Blows marks and analyzes the creature. The player learns the following about the creature: damage vulnerabilities, damage resistances, damage immunities, and condition immunities. In additional, whenever a mechanical creature misses the player, the player can use their reaction to make one unarmed melee attack against it. This benefit lasts until the end of a short or long rest.

In prior version published in the Tal'Dorei Campaign Setting, this ability cost one ki point to force a Constitution saving throw. On a failed save, the player must choose to learn one of the following: creature type, armor class, senses, highest saving throw modifier, lowest saving throw modifier, damage vulnerabilities, damage resistances, damage immunities, or condition immunities. The change was made because players felt "the limitations of unknown usefulness" frustrating. In both the campaign setting guide and the version published on Twitter, ability to immediately use a reaction on a missed attack was formerly the separate feat Preternatural Counter and could be used against any target, not just one targeted by Extract Aspects; it was learned at level 11 in the campaign guide and level 6 in the version published on Twitter.

Extort Truth
Beginning at level 6, after striking a creature with an unarmed attack, the player can spend one ki point to force a Charisma saving throw. The player knows if the creature fails or saves, and on a failed save, the creature cannot deliberately lie and all Charisma checks directed at them are made with advantage for up to ten minutes. The creature is aware of the nature of the effect and thus can avoid speaking or answer evasively.

In the prior versions published in the Tal'Dorei Campaign Setting and on Twitter, this ability required landing two or more attacks in one round (not required to be unarmed attacks) and lasted up to one minute.

Mystical Erudition
At level 6, the player learns on language of their choice and gains proficiency in one skill of their choice of Arcana, History, Investigation, Nature, and Religion. If they already have proficiency in one of those skills, they may instead choose to double their proficiency bonus for that skill. They gain an additional language and additional proficiency (or again doubling an existing proficiency) from that skill list at level 11 and again at level 17.

In the prior versions published in the Tal'Dorei Campaign Setting and on Twitter, this ability was first gained at level 3, then again at levels 11 and 17. In the campaign setting, there was no option to double an existing proficiency, and in the version published on Twitter, the option was available only at level 11 and 17.

Mind of Mercury
Beginning at level 11, if the player has already used their reaction, they may spend one ki point per turn to take an additional reaction.

In the prior version published in the Tal'Dorei Campaign Setting, the ability was learned at level 6 and allowed the player a number of reactions per round equal to their Intelligence modifier (minimum of 1) by spending one ki point per reaction beyond their first and limiting them to one reaction per trigger. It additionally allowed players to spend one ki point to gain advantage on an Investigation check. The reaction ability was felt to be prone to abuse, and the version of the subclass published on Twitter added a limitation of one per turn. Furthermore, to make the class more competitive with other monk subclasses and to offer an earlier combat ability, the Twitter version made Mind of Mercury available at level 11 instead.

Debilitating Barrage
Beginning at level 17, after striking a creature with an attack granted by Flurry of Blows, the player may spent three ki points to make the creature vulnerable to a damage type of choice for one minute or until the creature takes that damage type. Once affected by this ability, a creature cannot be affected again for 24 hours.

In the prior version published in the Tal'Dorei Campaign Setting, this ability required striking the creature with three or more attacks in a single round. Spending the three ki points, forced the creature to make all attack rolls at disadvantage until the end of the player's next turn and to additionally make a Constitution saving throw, suffering the damage vulnerability on a failure. The campaign setting guide additionally stated that the vulnerability does not affect a creature who has resistance or immunity to the chosen damage type, and this fact would be revealed to the player only after the type is chosen.