Monks who study the Way of the Monster learn how to read their opponents and learn to mimic their abilities with their ki. Masters of this way can harness the power of the most destructive creatures from around the multiverse.

Study your Enemy

At 3rd level you gain the ability to copy the abilities of monsters you see.

After witnessing a non-humanoid creature use an action to attack or cast a spell, you can spend 1 hour to meditate on what you saw to learn how to mimic it's properties. You must have seen the creature use the action within 24 hours before meditating. You can learn multiple actions during 1 meditation session. You can learn a number of actions equal to your Wisdom Modifier (minimum of one), which don't have to be from the same creature as long as all actions were seen within the last 24 hours. This meditation can be done during a long rest. These actions that you learn are called Monstrous Abilities. You can know a number of Monstrous Abilities equal to your Monk Level + Your Wisdom Modifier. When you learn new Monstrous Abilities, you may forget any of the ones you already know.

To be able to use a Monstrous Ability you must spend ki points equal to half of the CR of the creature you learned it from (rounded up). If you attempt to learn a spell from this feature, your monk level must be equal to or higher than twice the level of the spell, the spell has to be an innate ability of the creature, and the spell must not have needed material components. The number of ki points required to cast a spell are 1 for cantrips, 2 for 1st level spells, 3 for 2nd level spells, 5 for 3rd level spells, 6 for 4th level spells, 7 for 5th level spells, 9 for 6th level spells, 10 for 8th level spells and 11 for 9th level spells. For attacks or spells you learn that require an attack roll, you use the same to hit modifier as your unarmed attack. For attacks or spells that require a save you use the DC that is used by the creature or you use your Ki Save DC, whichever is higher. All damage is the same as listed in the creature's stat block. 

Mimic your Enemy

At 6th level you gain the ability to learn how to copy the physical and mental prowess of the monsters you observe.

When you meditate to learn new Monstrous Abilities you can add the AC, ability score, any special sense the creature had, and/or the proficiency modifier to a skill the creature was proficient with. To learn these abilities, you must have seen a non-humanoid creature be in combat for at least 2 rounds or observe the creature acting normally for 1 hour. As long as you know these Monstrous Abilities abilities you can, as an action, replace your AC, ability score, or proficiency modifier, with one that you know. You can also add a special sense that you don't already have as an action from your list of Monstrous Abilities. You can drop any ability gained from this class feature at any time (no action required). 

When you have any of these Monstrous Abilities active you spend ki points and your maximum ki points are reduced by the CR of the creature you learned the ability from (minimum 1). You can not regain ki points lost from this ability anyway other than dropping the ability. When you drop the ability you gain ki points equal to the number spent and your maximum increases by the same amount.

Mirror Move

At 6th level, you can throw an attack right back at an enemy or attack together with an ally creature.

When you see a creature use an action that you would be able to learn through your Study your Enemy feature, you can use your reaction to use it right then and there, without having it be a Monstrous Ability you know. You can choose your target for this attack. You do need to spend the ki points necessary to use the attack as if it was one of your Monstrous Abilities. If you don't have enough ki to use the ability, then you waste your reaction and nothing happens.

Imitate your Enemy

At 11th level, you learn to more closely imitate creatures.

You can now add a creature's damage immunity, damage resistance, condition immunity, and any special trait the creature has to your Monstrous Abilities. You can only add 1 trait per Monstrous Ability. These effects fallow the same rules as your Mimic your Enemy feature as to how you can learn them and how much they cost to have active.

Become your Enemy

At 17th level you learn how to become the monsters you know.

You can, as an action, use this ability to change your shape as though you had cast Shapechange (no need for spell components) with the added restriction that you can only change into non-humanoid creatures you've seen before. The effects last for the full duration of the spell (no need for concentration) or until your form drops do to your form dropping to 0 hit points.

You may not use this ability again before gaining a long rest.