A Mystic is, technically, any practitioner of a tradition or philosophy associated with magical or spiritual development and powers. In /tg/ media, a Mystic most often refers to a member of a particularly exotic philosophy who derives unusual powers from it; these are most commonly either a "non-standard" or "oriental" form of divine, psionic, elemental or (rarely) arcane magic or, much like the Witch, may be a class that uses powers normally split into different categories of magic.
In Dungeons & Dragons
Dungeons & Dragons has covered a lot of different spellcasting grounds in its time, and the Mystic is no exception. However, since D&D tends to rely heavily on the "Magic A is Magic A" trope, where magical traditions are very strictly defined, the Mystics have had... problems... catching on.
Appearing in the Dungeons & Dragons Rules Cyclopedia, the first ever D&D Mystic was a renamed Monk; a warrior trained in esoteric, spiritual traditions that granted him or her supernatural physical abilities. One of the earliest cases of Multiple Ability Dependency, a Mystic required a minimum of 13s in Wisdom and Dexterity to qualify for the class, and used Strength and Dexterity as its primary ability requisites. It was completely forbidden from using armor or even magical protection - no Rings of Protection, no Cloaks of Armor, nothing that artificially enhanced its AC (well, technically lowered, BECMI did use THAC0). Fortunately, its AC was improved by 1 step each time it gained a level, going from 9 at 1st level to -6 when it hit its maximum level of 16. It could use weapons, but its Martial Arts class feature saw its unarmed strikes grow increasingly potent with level, from 1d4 at level 1 to 3d12 at level 16, plus it improved to function as magic weapons at different levels (silver weapon at level 2, +1 at level 5, +2 at level 8, +3 at level 11, +4 at level 14, +5 at level 16).
The Mystic has access to some of the Fighter's combat manuevers, the Thief's Find Traps, Remove Traps, Move Silently, Climb Walls and Hide in Shadows, and an affinity for acrobatics. It also gained assorted special abilities as it leveled up; Awareness (only surprised on a 1) at 2nd level, Heal Self as per Cure Wounds 1/day at 4th level, Speak With Animals at 6th level, Resistance (halve spell & breath weapon damage, quarter it on a successful save) at 8th level, Speak With Anyone at 10th level, Mind Block (immune to ESP, Hold, Slow, and magical Charms/Quests/Geases) at 12th level, Blankout (become invisible to everyone, no save, for 1 round/level) 1/day at 14th level, and Gentle Touch (on a touch, cast Charm, Cureall, Death, Quest or Paralysis) 1/day at 16th level.
A slightly more famous Mystic arose in the Dragonlance setting. Due to a novel-based setting shakeup that saw the gods bugger off again after finally coming back in 1st edition, the Mystic was introduced as a more "free-wheeling" and low-powered alternative to the Cleric.
The Dragonlance Mystic was revised for 3.5e as a spontaenous caster that relied on the Cleric spell list, much like the Favored Soul. Unlike the Favored Soul, the Mystic had poor Reflex saving throws, no energy resistances, no wings at level 17, no proficiency with their deity's favored weapon, and no other Favored Soul class features. Unlike the Cleric, however, they had no ability to turn/rebuke undead, no heavy armor proficiency, no extra bonus language options... okay so what class features DO they have? None. They have literally no class features except access to a single Cleric domain. It's like Margaret Weiss just took the Favored Soul, stripped all of its class features out, didn't put anything back in except a single Cleric domain, and called it a day. What the fuck, MW?
Mystic is class introduced in Path of War - Expanded. Like Swordsage, it focuses on the openly supernatural disciplines. By default, it's the only class with the Elemental Flux and has special abilities that make that discipline more effective, as well as abilities that make them magical. The downside is you are never fully in control of what Manuevres you have available, as several of them come to you randomly in combat, and when you can't gain any more you reset in an explosion of arcane energy. Probably what Arcane Swordsage would have been if it was an actual class.
It has three archetypes which change the class in a fairly major way
Aurora Soul: Much closer to a monk / fighter type, than the insane gish mashup that is baseline Mystic, loses bonus feats, and the ability to make magic items in exchange for powerful Unarmed attacks, and a permanent WIS bonus to Touch / Flat-Footed / Regular AC.
Gunsmoke Mystic: Does what it says on the tin, you use guns as a method for delivering manuevres, trades away bonus feats and two rather powerful anti-magical abilities in exchange for essentially free, unlimited ammo, Gun-Fu, and an improved, and scaling, version of Rapid Reload that eventually stops people from AoOing you on reloading or firing in melee range. Is also exceptionally front loaded, gaining several extra abilities at early level, without replacing others.
Knight Chandler: The real treasure was the Friends we made along the way, the class. Trades out the Mystic's animus feature, glyphic area buffs, and natural anti-magic for a lot of slowly building, but quite powerful buffs. Emphasises the team-player aspects of the Mystic above and beyond the Glyphic area buffs.
Mystic is one of the core classes in Starfinder, a wisdom based caster class dealing in the subtler forms of magic. The mystic spell list is full of psionic stuff like esp, telepathy, and telekinetics, as well as powers of life and death such as healing. The more spectacular wizardish spells like making things spontaneously explode or summoning otherworldy terrors to claw at your enemies are more the providence of Witchwarpers.
In 5th Edition
Initially, the Mystic in Dungeons & Dragons 5th Edition was a sample class released for playtesting as part of the Unearthed Arcana: Psionics article. This class seems to have replaced the Psion as the core psionics-using class, and in fact seems to be able to mimic several psionic classes of former editions, depending on which Psionic Order one chooses to follow. Only two are presented in the UA article, but two others are noted as planned. On that same note, the article only includes progression up to 5th level.
The Order of the Awakened Mystic is closest to the iconic Psion, with access to the disciplines of Conquering Mind (altering thoughts), Intellect Fortress (psionic counter-attacks), and Third Eye (enhanced sensory abilities). As innate abilities, it gets Mind Thrust (psychic blasting), Psychic Mind (telepathic communication) and Object Reading (postcognition).
The Order of the Immortal Mystic, on the other hand, is closer to the Battlemind or Soulknife, being a psychic who focuses on channelling psionic energy through their body. They get martial weapon & shield proficiency for free, Durable Mind (concentration can't be broken by taking damage) and Psionic Regeneration (regain missing hit points equal to half of Mystic level) for free, and their disciplines are Celerity (super-speed), Iron Durability (super toughness) and Psionic Weapon (channel psionic energy through a weapon to enhance it in various ways).
It eventually got bumped up to a 10-level class in the February 2016 UA article, which tweaked the disciplines around and made it more flexible.
The March 13th 2017 Unearthed Arcana finally saw the Mystic Mark 3, which brings the class to full 20th level. This brings it to full fruition; aside from having six subclasses, all Mystics now gain access to both Disciplines and Talents. Psionic Talents are essentially the Mystic's equivalents of cantrips, being at-will powers that the Mystic doesn't need to fuel with psi-points. Disciplines are psionic "stances", for lack of a better word; a Mystic can engage in one Discipline at a time, granting access to its passive bonus and allowing the Mystic to spend psi-points to activate various powers tied to that Discipline. This version also has six different subclasses; Avatar, Awakened, Immortal, Nomad, Soul Knife and Wu Jen. Most Mystic subclasses have "associated" Disciplines, forming a list they can take bonus Disciplines from, but all Mystics can freely take any and all of the various Disciplines. In fact, of six subclasses, only the Soul Knife lacks the Bonus Disciplines feature.
All Mystics gain the class features Mystic Recovery (spend a bonus action after using psi points to regain hi points) and Telepathy 120 feet at 2nd level. They then pick up Strength of Mind (switch Proficiency in Wisdom saving throws to instead use your choice of Strength, Dexterity, Constitution or Charisma Proficiency whenever you complete a short or long rest) at 4th level. 8th level grants them Potent Psionics (can, once per turn, deal +1d8 (+2d8 if level 14+) psionic attacks with a weapon attack and also add +Int modifier damage to all psionic talent attacks) at 8th level. At level 10, they gain access to Consumptive Power (can burn HP instead of PP for a psionic attack once per long rest). Level 11 provides access to Psionic Mastery (gain a pool of bonus psi points that can be used to power Disciplines, breaking several normal rules). Finally, at level 20, Psionic Body grants Resistance to physical (slashing, bludgeoning, piercing) damage, immunity to aging, disease, poison and the poisoned condition, and a 55% chance of spontaneously resurrecting 1d3 days later if ever you are killed.
The Order of the Avatar subclass is essentially the 4e Ardent brought back to life in 5e. At 1st level, it picks up free proficiency with medium armor and shields via Armor Training, whilst its primary bonus is projecting a psionic aura that grants stacking buffs based on the Avatar's level, though the aura only reaches 30 feet and is only active when it isn't incapacitated. Avatar of Battle (3rd level) grants +2 to Initiative rolls. Avatar of Healing (6th level) means allies within the Avatar's aura gain bonus hit points when subjected to healing psionic powers. Finally, Avatar of Speed (14th level) grants allies within the aura the ability to Dash as a bonus action. The Avatar's associated disciplines are:
- Crown of Despair
- Crown of Disgust
- Crown of Rage
- Mantle of Command
- Mantle of Courage
- Mantle of Fear
- Mantle of Fury
- Mantle of Joy
The Order of the Awakened subclass is essentially a blending of the Clairsentient (psychic divination) and Telepathy focused psions from editions long past. At 1st level, it gains two bonus skill proficiencies from a list consisting of Animal Handling, Deception, Insight, Intimidation, Investigation, Perception and Persuasion, via its Awakened Talent. 3rd level gives it the Psionic Investigation power, which is basically the old Postcognition power (touch something, learn various things about its history) as a once-per-short-rest ability. 6th level lets it use Psionic Surge, immediately ending its current psionic focus in order to use a Discipline or Talent against which the target suffers Disadvantage on their saving throw. Finally, at level 14, Spectral Form allows the Awakened to enter a ghostly state (halve speed, gain Incorporeal Movement and Resistance to Physical Damage) for 10 minutes once per long rest. The Awakened's associated disciplines are:
- Aura Sight
- Intellect Fortress
- Mantle of Awe
- Psychic Assault
- Psychic Disruption
- Psychic Inquisition
- Psychic Phantoms
- Telepathic Contact
The Order of the Immortal subclass is based on the Psychometabolism powers of old, using psionic energy to bolster its physical form in various ways. Its 1st level feature, Immortal Durability, grants it +1 max hit point per Mystic level and makes its unarmored base AC equal 10 + Dex modifier + Con modifier. At level 3, so long as it's still standing (1+ hit points), it gains temporary hit points equal to its Int modifier at the start of each turn, courtesy of Psionic Resilience. Level 6 brings access to the Surge of Health feature, where the Immortal can choose to end their current psionic focus as reaction to taking damage in order to halve that damage, although doing so keeps them from reusing their psionic focus until they take a short rest. Finally, at level 14, Immortal Will lets an Immortal at 0 hit points immediately spend 5 psi points to regain Mystic level + Con modifier hit points - and keep in mind, by level 14, a Mystic has 64 psi points to spend. The Immortal's associated disciplines are:
- Adaptive Body
- Bestial Body
- Brute Force
- Corrosive Metabolism
- Giant Growth
- Iron Growth
- Psionic Restoration
- Psionic Weapon
The Order of the Nomad subclass is another throwback to past classifications of psion, specifically the Psychoportation school of psionic teleportation, although it also blends in aspects of Clairsentience. Its 1st level feature, Breadth of Knowledge, gives it two free-floating proficiencies it can put in any combination of two skills and/or two tools whenever it completes a long rest. Level 3 gives it Memory of One Thousand Steps, where, once per short rest, the Nomad can use a reaction to being hit by an attack to teleport to an unoccupied space that it occupied last turn, which incidentally also nullifies the damage from the triggering attack. At level 6, Superior Teleportation means that the Nomad boosts the range of all psionic teleportation powers by +10 feet. Finally, level 14 gives it Effortless Journey, which lets it forfeit moving normally during a turn to instead make a 30ft teleport. The Nomad's associated disciplines are:
- Nomadic Arrow
- Nomadic Chameleon
- Nomadic Mind
- Nomadic Step
- Third Eye
The Order of the Soul Knife subclass is, you guessed it, the 5eification of the original 3rd edition Soulknife. A fairly faithful rendition of the original, its primary focus is on stabbing shit with psionic blade attacks. Not only does it gain free proficiency with medium armor and martial weapons (Martial Training), but its iconic ability is Soul Knife, which lets it conjure scintillating energy blades around both hands or wrists, which are great for stabbing people in their soul (light, finesse martial melee weapon that does 1d8 psychic damage on a hit) but prevents you using your hands for anything else whilst you have them activated. Conjuring or dismissing them takes a bonus action. Needless to say, its other subclass features focus on bolstering its mind-stabbing powers: Hone the Blade (3rd level) lets you spend psi-points for a boost to attack rolls & damage rolls (2 points for a +1, 5 points for a +2, 7 points for a +4) that lasts 10 minutes. 6th level's Consumptive Knife means you regain 2 spent psi points whenever you kill somebody with your Soul Knife. Finally, at level 14, you get the Phantom Knife ability, which lets you use your action to make an attack with your Soul Knife that ignores the target's AC and instead targets AC 10. Unfortunately, however, it doesn't get even a single discipline unto itself.
The Order of the Wu Jen subclass brings back the "Oriental Wizard" of old by absorbing the Psychokinetic, focusing on mentally manipulating the various elements. Hermit's Training (1st level) grants it free proficiency in any two skills from a list consisting of Animal Handling, Arcana, History, Insight, Medicine, Nature, Perception, Religion, or Survival. Elemental Attunement (3rd level) lets it spend +1 psi point when making an elemental psionic attack to ignore the target's Elemental Resistance to that attack, though this can't be done if you've already maxed out your psi limit on the attack. Arcane Dabbler (6th level) gives it three low-level (1-3) Wizard spells, which it can cast as up to 5th level spells by using psi points. Finally, Elemental Mastery (14th level) lets it boost the Damage Resistance it gains from a psionic focus to Damage Immunity for 1 round as a reaction to taking damage of the appropriate type. The Wu Jen's associated disciplines are:
- Mastery of Air
- Mastery of Fire
- Mastery of Force
- Mastery of Ice
- Mastery of Light & Darkness
- Mastery of Water
- Mastery of Weather
- Mastery of Wood & Earth
There is actually a surprisingly official-looking homebrew expansion to this in the from of a PDF: https://drive.google.com/file/d/0B2bXgsHg7PtJNXdRdjdaUDhRM2M/view?fbclid=IwAR29j5mT2gBK87ZWAbDoelfk-k1YQQuZMgDwvUXYbYQjW6FU1gZu0U10eGU Additions include a list of disciplines for the Soul Knife, a new subclass for the fighter and rogue each, several psionic-related feats, new "magic" items, and even several new psionic mosnters.
In April 2020, a UA was released called "Psionic Options Revisited", which has more or less de-canonized what little ground the class had. In the article, they state that people liked the idea of psi-focused themes, but hated that the class cribbed off other classes for its features. Add onto this the overall complexity and power of the class as a whole, and WoTC decided to toss the entire class to the wayside, instead using psi-focused subclasses and feats to satisfy the desire for psi-classes.
In Warhammer 40,000
For more information, see here: Mystic (40k)