Path of Fundamental Chaos

The Path of the Fundamental Chaos is the barbarian subclass of Ashton Greymoore. It is implied to be connected to dunamancy, as Ashton's head injury was healed with, among other things, a vial of dunamantic potion.

Chaos Burst
Upon choosing this path at level 3, a Fundamental Chaos barbarian can choose to deal extra damage on an attack with a chaos burst. This allows them to add 2d4 damage to an attack with a melee weapon a number of times per day equal to their proficiency bonus. Damage type depends on a rolled die and includes lightning, force, cold, fire, and psychic.

Rage Type
Also at level 3, the rage type is determined randomly by rolling 1d4 whenever the barbarian begins their rage:
 * Orbital Decay (Gravity): When raging, gravity seems to have more of a pull for everyone in the room (including allies). Enemies within 15 feet are pulled up to 10 feet toward the barbarian and suffer disadvantage on attacks against creatures other than the barbarian.
 * Temporal Morass (Time): The barbarian may choose a target, who must make a Constitution saving throw to avoid having their speed halved and lose their reaction.
 * Space: When the barbarian makes a melee attack, they can open a portal to a space within 60 feet that they can see and attack a creature as though the barbarian were in that space.
 * Luck (Potential): Each ally within 15 feet of the barbarian can add a d4 to attack rolls and saving throws.

Fundamental Force
Fundamental Force allows a raging barbarian to spend a use of Chaos Burst, triggering a different effect for each type of rage.
 * Hyper Rage (Time): The barbarian can make an additional attack as part of the Attack action.
 * Density Well (Gravity): The barbarian cannot be moved against their will or knocked prone, and when they hit a creature with a melee weapon attack it must make a Strength saving throw or be knocked prone.
 * Space: The barbarian can open two portals, one next to themselves and another at a point within 60 feet. They may attack a creature through this portal as if the two portals are adjacent to one another. This attack deals additional damage equal to one roll of a Chaos Burst die.

Behind the scenes
Taliesin Jaffe asked Matthew Mercer if he could homebrew a barbarian class, as he was dissatisfied the Path of Wild Magic barbarian from Tasha's Cauldron of Everything, in which your powers of chaos become more controlled as you level up. He wanted a class that was more chaotic and unpredictable. The process of creating the class was collaborative.