Those monks who have encountered the dark forces in the Shadowfell or have crossed paths with the shadar-kai may have had their outlook on combat changed.  Affecting a target's mind can be even more debilitating than damaging their body.  An enemy that loses the will to fight is eternally defeated.  Confusing the senses with illusions, briefly pushing their body into an ethereal-like state, and summoning weapons of pure shadow are all traits that monks can acquire by diving in the Way of the Sworn Sorrow.

Wavering Form

Starting when you choose this tradition at 3rd level, you can briefly enter into an ethereal-like state whenever you spend ki points on an ability.  Whenever you spend ki points, you can gain resistance to all forms of damage except for force and can move through objects and creatures as if you were under the effects of the etherealness spell until the beginning of your next turn.  If you end your turn inside of an object or creature, you take 1d10 force damage and are shunted to the nearest unoccupied space.  You can do this a number of times equal to your proficiency bonus, unless you spend 1 additional ki point whenever you spend ki to do so.

Modus of the Phantom

At 6th level, you gain access to specialized techniques that channel the aspects of Shadowfell creatures:

• Phantasmal Blade (3 ki points).  As a bonus action, you can summon a blade of shadowy energy to harm your foes.  It counts as a monk weapon with which you are proficient. You can use your ki attack modifier for attack rolls.  It deals psychic damage equal to two times your martial arts die on a hit plus your Wisdom modifier and has the finesse, light, and thrown properties (range 20/60). In addition, when you use the sword to attack a target that is in dim light or darkness, you make the attack roll with advantage.  If you drop the weapon or throw it, it dissipates at the end of the turn. Thereafter, while the effect persists, you can use a bonus action to cause the sword to reappear in your hand.  This effect lasts for 1 minute.
• Phantasmal Images (3 ki points).  As a reaction whenever you are struck by an attack, you can split your form into three illusionary duplicates that mirror your movements.  Until the effect ends, the duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates.  Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.  If you have three duplicates, you must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, you must roll an 8 or higher. With one duplicate, you must roll an 11 or higher.  A duplicate's AC equals 10 + your Dexterity modifier + your Wisdom modifier. If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects. The spell ends when all three duplicates are destroyed.  A creature is unaffected by this effect if it can't see, if it relies on senses other than sight, such as blindsight, or if it can perceive illusions as false, as with truesight.  This effect lasts for 1 minute.
• Phantasmal Vision (2 ki points).  Whenever a creature fails the saving throw for your Stunning Strike ability, you can spend 2 additional ki points to scramble their perception of reality.  You craft an illusion that takes root in the mind of the stunned creature.  You create a phantasmal object, creature, or other visible phenomenon of your choice that is no larger than a 10-foot cube and that is perceivable only to the target for the duration.  The phantasm includes sound, temperature, and other stimuli, also evident only to the creature.  The target can use its action to examine the phantasm with an Intelligence (Investigation) check against your ki save DC. If the check succeeds, the target realizes that the phantasm is an illusion, and the effect ends.  While a target is affected by the technique, the target treats the phantasm as if it were real. The target rationalizes any illogical outcomes from interacting with the phantasm. For example, a target attempting to walk across a phantasmal bridge that spans a chasm falls once it steps onto the bridge. If the target survives the fall, it still believes that the bridge exists and comes up with some other explanation for its fall - it was pushed, it slipped, or a strong wind might have knocked it off.  An affected target is so convinced of the phantasm’s reality that it can even take damage from the illusion. A phantasm created to appear as a creature can attack the target. Similarly, a phantasm created to appear as fire, a pool of acid, or lava can burn the target. Each round on your turn, the phantasm can deal 1d6 psychic damage to the target if it is in the phantasm’s area or within 5 feet of the phantasm, provided that the illusion is of a creature or hazard that could logically deal damage, such as by attacking. The target perceives the damage as a type appropriate to the illusion.  This effect lasts for 1 minute.

Psychic Redouble

Beginning at 11th level, whenever you use your Stillness of Mind ability to end one effect on yourself that is causing you to be charmed or frightened, you gain advantage on your next Intelligence, Wisdom, or Charisma saving throw.

Adroit Phantom

At 17th level, you have further developed your ability to tap into phantasmal power and gain the following techniques:

• Phantasmal Slayer (4 ki points).  Whenever a creature fails the save for your Stunning Strike ability, you can spend 4 ki points to drastically alter its perception of reality.  The target becomes frightened for 1 minute. At the end of each of the target's turns before the effect ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the effect ends.
• Phantasmal Spy (6 ki points).  As an action you can create an illusionary double of yourself that you control  The double is lasts for 1 minute.  You can use your action to move your illusory double up to twice your speed and make it gesture, speak, and behave in whatever way you choose.  You can see through its eyes and hear through its ears as if you were located where it is. On each of your turns as a bonus action, you can switch from using its senses to using your own, or back again. While you are using its senses, you are blinded and deafened in regard to your own surroundings.  Whenever your utilize your Empty Body ability at 18th level and beyond to become invisible, you can spend 2 additional ki points to summon this illusionary duplicate.