Monks who choose this path are the kind of beings who like play with their enemies. They sacrifice they material and mortal self to twist the mind and body of their enemies using the fear they create from the encounter. They are the one who you refer to when you say "exception proves the rule" as Monks on the way of the phantom tend to have evil nature usually connected to a tragedy. For example a landlord made his/her and their family's life suffering making them do the same as they thinks they have the right to do it.

Phantom self

You use your ki to bring fear on your enemies. When you choose this subclass at level 3 you gain proficiency in stealth (if you don’t have it already) and you can cast the Cause Fear and Blindness/Deafness spells for 1 ki point each as a bonus action. You can also use additional ki points to cast these spells at higher levels. Your appearance also changes to represent your will of leaving your mortal self. You choose up to 3 parts of your body that become phantom-ised or roll for it using the Phantom Appearance table. You describe their glowing color. It can also have addition markings of your choice but the colors has to be close to eachother.

Casting spells. The spell's level increases by 1 for each additional ki point you spend. For example, if you are a 5th-level monk and use Sweeping Cinder Strike to cast burning hands, you can spend 3 ki points to cast it as a 2nd-level spell (the discipline’s base cost of 2 ki points plus 1). The maximum number of ki points you can spend to cast a spell in this way (including its base ki point cost and any additional ki points you spend to increase its level) is determined by your monk level, as shown in the Spells and Ki Points table.

Spells and Ki points
Monk Levels	Maximum Ki Points for a Spell
5th—8th	3
9th—12th	4
13th—16th	5
17th—20th	6

 

Phantom Appearance
D6	Description
1	Your left arm, right leg and half of your head.
2	Both of your hands and left leg.
3	Both of your arms, and your heart(the glowing would be visible through your skin).
4	Both your legs and eyes with a slashing mark across your face.
5	Your right arm, heart, and eyes with a slashed mark across your face.
6	Spots on your entire body of your choice.

Shadow Step

Not every surprise is pleasant. When you reach level 6 as a bonus action you can spend 2 ki points to throw a dark ball anywhere in your movement range and teleport there without triggering any reaction (you can only teleport if the ball isn’t further than your remaining movement). You can also use it as a reaction to dodge enemy attacks. In that case in your next turn your movement lowered by the same amount it would have required to move to that spot. Additionally if you are in 10ft from an enemy when teleporting every enemy that saw you reappear in that range must succeed a wisdom saving throw. On a fail they knocked prone. On a success they aren’t knocked prone and become immune for this effect for 1 hour.

Weapon of Fear

You manifest the fear of your enemies. At level 17 in place of one of your attacks from your Extra attack feature you can use your bonus action to spend 5 ki points to attack every enemy within 30ft of you with the same melee attack. You roll the attack roll for each enemy that is in your reach in this way and they deal necrotic damage. At level 20 you can use 3 addition ki points to teleport to an enemy that was effected by this and attack again.

Living Between Realms

Not even reality can stop you. As you grab onto reality itself you rip a hole open and as you vanish from the world you enter a realm between realms. At level 11 as an action you can use 4 ki points to vanish from where you currently are. While you are gone your movement and jump distance doubled and you are considered to be invisible with no way for enemies to detect you and you can move through objects as they were difficulty terrain. Additionally allies’ and enemies’ effects and spells – areal included -  cannot reach or harm you and vice versa . This effect ends at the end of your next turn or when you make any kind of attack action. In addition you can spend 2 additional ki points to stay in this effect until your next turn’s end.