A monk following the Way of the Living Weapon has managed to master the art of breaking from their true form and learning to push their new form to untold lengths, honing their body as a smith sharpens a sword. Its traditions are drawn from cultures of shapechangers, lycanthropes, and creatures who fight with tooth and claw. 

Mutable Forms

At 3rd level, you gain the ability to transform from your true humanoid form into an eldritch, mutable form. The appearance of the form you take is up to you. You gain the shapechanger creature type in addition to the humanoid creature type. Using your bonus action, you can spend 2 ki points to change into a mutable form. When you do so, each creature of your choice that you can see within your reach must succeed on a Dexterity saving throw or take damage equal to 2 rolls of your Martial Arts die.

While you are maintaining a mutable form, the following applies to you:

• Your Martial Arts damage die increases for unarmed strikes. You can roll a d6 in place of the normal damage for your unarmed strike. This die changes to a d8 at 5th level; a d10 at 11th level; and a d12 at 17th level. This increased damage can only be applied to an unarmed strike, and not to a monk weapon.
• You can use your Wisdom modifier in place of your Strength or Dexterity modifier when making ability checks, attack rolls, and saving throws that rely on Strength or Dexterity.
• You may use your bonus action to change your size. Your size can be up to one size larger than normal, and your reach is 5 feet greater than normal.
• While maintaining one of your mutable forms, you have resistance to non-magical bludgeoning, piercing, and slashing damage.
• You gain darkvision out to a range of 60 feet. If you already possess darkvision, the range of your darkvision while transformed increases by an additional 60 feet.
• You gain a bonus to your AC and damage rolls and equal to your Proficiency bonus.
• When you use your Flurry of Blows, you can make one additional attack with your unarmed strikes. At 6th level, this increases to two additional attacks, and three attacks at 11th level.

You can maintain a mutable form for a number of hours equal to your proficiency bonus, after which you change back into your true form, unless you expend another use of this feature. This transformation ends early if you are incapacitated or die. You can't wield weapons or do anything that requires manual precision, such as using tools or magic items or performing the somatic components of a spell while maintaining a mutable form.

Retributive Body

Also at 3rd level, you can use your reaction to deflect or absorb the attack when you are hit by a melee attack. When you do so, the damage you take from the attack is reduced by 1d10 + your Dexterity modifier + your monk level. If you reduce the damage to half the original damage or less, you can make one unarmed strike as part of the same reaction, or spend 1 ki point to Flurry of Blows instead.

Swell of the Soma

At 6th level, after spending time honing how to transform in and out of your mutable form, you have learned to push your shapeshifting capabilities further than before. When you transform into your mutable form or while maintaining one as a bonus action, you may spend an additional ki point to have the following apply to you:

• You regain hit points at the start of your turn if you have at least 1 hit point. The amount regained is equal to one roll of your Martial Arts die + your Con modifier. If you take damage, this feature ceases to function until the start of your next turn.
• You may use your bonus action to change your size. Your size can be up to two sizes larger than normal, and your reach is 10 feet greater than normal.
• You gain a climbing and swimming speed equal to your walking speed, which increases by 10 feet.
• Your unarmed strikes now score a critical hit on a roll of 19, or 20.
• Starting at 7th level, when you use Stillness of Mind to end a condition affecting you, you may choose from the following additional conditions: blinded, deafened, grappled or poisoned. At 11th level, you may also choose from the paralyzed, restrained, and or stunned conditions.

Adaptive Anatomy

At 11th level, you have nearly managed to push your shapeshifting to its breaking point. When you transform into a mutable form, creatures within 60 feet of you that can see you must succeed on a Wisdom saving throw against your Ki Save DC or be frightened of you for 1 minute. Creatures frightened in this way can make additional Wisdom saving throws to end this effect at the end of each of their turns. In addition, when you transform into your mutable form or while maintaining one, you may spend an additional ki point to have the following apply to you:

• When you make an unarmed strike, you can choose to have the target make a Constitution saving throw against your Ki Save DC or be affected in one of these ways: pushed up to 15 feet away, grappled, knocked prone, or become poisoned until the start of your next turn. Creatures poisoned in this way take 1d4 force damage at the start of their turn and can make an additional Constitution saving throw at the end of their turn to end the effect. 
• While maintaining one of your Lycanthropic forms, you have immunity to non-magical bludgeoning, piercing, and slashing damage.
• Choose one damage type from the list when you activate this ability: acid, cold, fire, force, lightning, poison, or thunder damage. You gain immunity to the chosen type.
• Your unarmed strikes now score a critical hit on a roll of 18, 19, or 20.
• You can use your reaction to gain advantage on attack rolls or ability checks that rely on Strength or Dexterity until the end of your turn.

Mastery of the Living Weapon

At 17th level, your mastery to your mutable form is complete, allowing you to unleash its full potential. As a bonus action, you can spend 5 ki points to transform into a mutable form and benefit from the Swell of the Soma, Adaptive Anatomy, and Mastery of the Living Weapon abilities all at once. 

While you are transformed in this way, you gain the following benefits:

• You may use your bonus action to change your size. Your size can be up to three sizes larger than normal, and your reach is 15 feet greater than normal.
• You deal double damage to objects and structures.
• Whenever you use the Extra Attack feature to attack twice, you can instead attack three times if all the attacks are made with your unarmed strikes.
• When a creature within 20 feet of you is reduced to 0 hit points, you can use your reaction to regain ki points equal to your Wisdom modifier (minimum 1).