Use your instinct and knowledge of the way living beings are constructed to hinder your opponents, strike true in the places where it hurts, and dart around the battlefield

Level 3: Vital strikes

When critting on an opponent due to a natural 20 or other means, if you roll Max damage on one or more of your damage dice, roll for damage again on the dice you rolled Max damage on, adding up the damage until you fail to roll Max damage.

Eg: when critting with a dagger, roll 2 d4 as per usual, if you get a 2 and a 3, add your damage modifier and continue as usual.

however, if you got a 4 on any of the two dice, since it is the highest you can roll on that particular dice, you may continue stacking damage rolls, eg: you roll a 1 and a 4,  that's 5 damage, roll 1d4 again, another 4, that's 9 damage, roll again, continue until you fail to roll a 4, then add your modifiers and continue

This effect is even more potent when rolling multiple Max damages, as you get to reroll as many dice as you roll Max damage on, eg: 2 fours, roll 2d4 again, 2 fours again, roll 2d4 again, a 2 and a 4, roll 1d4, continue until you run out of luck

Level 6: Peeling the scales

from now on, rolling an attack roll and landing on a number 10 higher than the opponent's armour class is a critical hit, for example, if your opponent has an armour class of 15, and you roll a 25, that's a critical, remedying the reliance on luck to hit properly

while in rage, critting also induces a coin flip, where if you win the coin flip (by calling a side and having that side land face up), you will reduce the target's AC by 1d4 - 1

at level 10 this becomes 1d4 provided you win the coin flip

at level 14 this becomes 1d4 with no necessary coin flip

Level 10: Let the blood flow

While in rage, you are capable of entering a 'flow' by dealing damage equal to your (levels in barbarian x 3) + 25, so for example, you'd need to do 55 damage at level 10 while in rage to enter a flow, 58 damage at level 11, and 85 damage at level 20, after dealing the required damage, entering a flow is a bonus action

a flow lasts for a max of 5 turns, but it is easy to be knocked out of one, as taking damage equal to more than a quarter of your health in a single attack will knock you out of it, and not dealing damage during your turn will also end your flow, getting knocked out of your flow means you have to deal damage to build it back up again, but by re-entering a flow, you get your instinct charges back

while in a flow, you gain access to a multitude of different things, including:

an extra bonus action, +10 feet to your movement speed, +2 dex, the ability to crit on a 19, and access to instinct. at level 10 you have 3 charges of instinct, at 14 you have 4, and at 20 you get 5. instinct is a powerful resource that allows you to use exceptional abilities and techniques. some techniques will include a save, where the DC is 8 + your dex or str bonus + your proficiency bonus, your options include:

Lightspeed reflexes, 1 instinct, reaction: when being attacked, use this to increase your AC by 3, before or after the enemy reveals their attack roll, but not after they reveal their damage roll, this can also be used to gain a +3 bonus to any saving throw

Million ton piercer, 1 instinct, free action: when attacking, you may use a charge of instinct to increase your attack bonus by 3 before or after your roll, which could lead to a critical hit, or just allow you to hit your foe

Ghost, 1 instinct, reaction: after dodging an enemy attack or succeeding a saving throw, move up to 30 feet without provoking opportunity attacks

Spiteful resilience, 2 instinct, reaction: when rolling a critical failure, you can increase the amount on the dice, even up to a 20, but it costs you health equal to the amount you increase it by

Ankle cutter, 2 instinct, bonus action: crouch down and quickly slice in a circle with your blades, dealing 3d6 damage and knocking the opponent prone for 3 turns unless they succeed a dex saving throw, prone inflicted by this method also makes them unable to get back up until the condition runs out on its own

Eucharist, 2 instinct, bonus action: with all this food around, why let it go to waste? if there is a corpse within 5 feet of you, you can choose to consume it, healing 6d6 + 1, 2, 3, 5, 10, or 15 depending on the size of the creature the corpse comes from (1 being tiny, 2 being small, 3 being medium, 5 being large, etc etc)

Cull the herd: 2 instinct, free action: after hitting an opponent in proximity of other enemies (5 feet), the secondary enemies must succeed a dex saving throw or take damage equal to the amount the main target took

Butterfly: 3 instinct, free action: if you are within 30 feet of an attacking enemy, immediately close the distance and interrupt their turn with your own.

Anatomical precision

3 instinct, bonus action:

for this turn and the next, every single one of your attacks that you land are guaranteed to be a critical hit, but take a -8 penalty to your attack, and a -4 penalty to your armour class

Ankle cutter

2 instinct, bonus action:

crouch down and quickly slice in a circle with your blades, dealing 3d6 damage and knocking the opponent prone for 3 turns unless they succeed a dex saving throw, prone inflicted by this method also makes them unable to get back up until the condition runs out on its own

Butterfly

3 instinct, free action:

if you are within 30 feet of an attacking enemy, immediately close the distance and interrupt their turn with your own.

Cull the herd

2 instinct, free action:

after hitting an opponent in proximity of other enemies (5 feet), the secondary enemies must succeed a dex saving throw or take damage equal to the amount the main target took

Eucharist

2 instinct, bonus action:

with all this food around, why let it go to waste? if there is a corpse within 5 feet of you, you can choose to consume it, healing 6d6 + 1, 2, 3, 5, 10, or 15 depending on the size of the creature the corpse comes from (1 being tiny, 2 being small, 3 being medium, 5 being large, etc etc)

 

Ghost

1 instinct, reaction

after dodging an enemy attack or succeeding a saving throw, move up to 30 feet without provoking opportunity attacks

Lightspeed reflexes

1 instinct, reaction:

when being attacked, use this to increase your AC by 3, before or after the enemy reveals their attack roll, but not after they reveal their damage roll, this can also be used to gain a +3 bonus to any saving throw

Million ton piercer

1 instinct, free action:

when attacking, you may use a charge of instinct to increase your attack bonus by 3 before or after your roll, which could lead to a critical hit, or just allow you to hit your foe

Spiteful resilience

2 instinct, reaction:

when rolling a critical failure, you can increase the amount on the dice, even up to a 20, but it costs you health equal to the amount you increase it by

 

Crippling critical

from now on, once per turn, when you land a crit while in rage, you will have a chance to cause a debilitating condition onto your opponent

on a crit while in rage, once per turn, roll a d12

1-8 = nothing

9 = exsanguination: on a successful proc, the opponent will start suffering heavy blood loss and have to make a constitution saving throw whenever it wishes to move or attack, failing the save causes the opponent to take 1d8 damage, this condition lasts until the opponent rolls a 20 on their constitution saving throw, until the opponent heals through any means, until the opponent lasts 3 turns without taking damage from any source, or until the opponent dies

10 = jagged wounds: on a successful proc, the opponent will be unable to heal for 2 turns

11 = collapsed lung: on a successful proc, the opponent will have their movement speed cut in half, and have disadvantage on saving throws

12 = torn tendon: on a successful proc, the opponent will no longer be able to use one of their limbs until they receive healing of some kind, the limb that is disabled is decided by the butcher, a leg means the opponent will have to make a dex save every time they want to move or go prone, and have disadvantage on any dex saving throws, and an arm means it must drop whatever it is holding in said arm, and have disadvantage on attack rolls with two handed weapons

while in flow, the same effects are possible, except instead of being limited to one condition per turn, you can cause as many conditions as you land critical hits, and instead of rolling a d12, you roll a d8

1-4 = nothing

5 = exanguination

6 = jagged wounds

7 = collapsed lung

8 = torn tendon