The Eideticist

The Eideticist, armed with split-second observation and photographic reflexes, charges into battle ready to mirror the ferocity and deadly prowess of their enemies. Through mere observation, an eidetic fighter may quickly find the vulnerabilities in an enemy's defenses or learn their techniques and turn the tide of a battle. The eidetic fighter welcomes a deadly opponent, knowing that with each blow they will becomes stronger in the face of adversity. 

Left Wide Open

When you choose this archetype at 3rd level, you learn to patiently watch your enemy's movements closely for patterns and strike them when they are most vulnerable. You may expend your action surge ability to grant advantage to all of your attacks this turn. If these attacks hit, they deal an additional die of weapon damage. 

The Art of Observation

Starting at 3rd level, you begin to notice the finer details of the people and things around you. As a result of your sharpening ability, you gain proficiency in the insight and perception skills. 

Analyze Technique

At 7th level, when a creature damages you with a weapon attack, you can use your reaction to analyze the blow to determine how best to avoid it in future. If you do, weapon attacks of this type made by that creature against you have disadvantage until the end of your next turn.

You can use this ability a number of times equal to your Intelligence modifier (minimum of 1). You regain all uses of this ability on a short rest.

Countermastery

Starting at 10th level, you've learned to mirror the attacks of your enemy. When you take damage from a melee weapon attack, you can use your reaction to learn the technique used to produce that much damage. Until the end of your next turn, your melee weapon attacks have the same number of damage dice as that attack as well as the same damage type. 

You may use this ability a number of times equal to your Intelligence modifier (minimum of 1). You regain all uses of this ability on a long rest. 

One Step Ahead

Starting at 15th level, when a creature you can see within 30 feet of you performs one of the following actions, you can use your reaction to gain the listed benefit. This effect lasts until the end of your next turn. 

You can use this ability a number of times equal to your Intelligence modifier plus one, at which point you must take a long rest to regain all uses of this ability.

Movement. If the creature uses its walking speed to move a distance greater than your current speed, or uses the dash action, your speed increases to the distance the creature moved.

Multiattack. If the creature uses the multiattack ability to make multiple weapon attacks, the number of attacks you can take with the attack action on your next turn increases to that number of attacks plus one.

Legendary Resistance. If the creature uses a legendary resistance, you gain advantage on the next saving throw you make.

Legendary Action. If the creature uses a legendary action, immediately afterwards you may move up to your movement speed and make an attack.

Cast a Spell. If the creature casts a spell of 3rd level or lower that does not require a material component, you can use your action to cast the same spell once before the end of your next turn. You always cast this spell at the lowest possible level.

Perfect Reflection

Once you reach 18th level, your ability to mirror your opponents strikes has become almost completely automatic, matching every attack with an identical counter attack. Whenever a creature damages you with a melee attack, you may use your reaction to immediately strike back with an identical attack. Make a weapon attack. On a hit, the attack deals the same amount and type of damage as the attack that you just received.

 You may use this feature once per short or long rest.