Eideticist

The Eideticist fighter has honed their skills of observation, coordination, and muscle memory to the point where they are able learn their enemies strengths and weaknesses through mere observation, and then mirror those strengths and turn them against their foe. When an Eideticist takes to battle, they are a blank canvas ready to absorb the abilities of their enemies, each attack made against them making them stronger.

Analyse Attack

Beginning when you choose this archetype at 3rd level, when a creature damages you with a weapon attack, you can use your reaction to analyse the blow to determine how best to avoid it in future. If you do, weapon attacks made by that creature have disadvantage. This effect lasts until the creature dies, you use this ability again, or you are incapacitated.

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.

Astute Combatant

At 3rd level, you get proficiency in insight, investigation, and perception. You can use your intelligence modifier when making ability checks using these skills.

Pinpoint Vulnerability

From 7th level, you are able to hone your perception and identify an enemies weakness, delivering a potentially deadly blow. You can use the action granted by your Action Surge ability to mark the weak spot of a creature you can see within 30 feet of you. The next attack that hits the creature marked this way counts as a critical hit.

Countermastery

At 10th level, when you take damage from a melee weapon attack you can use your reaction to learn the techniques needed to produce that much damage. Until the end of your next turn, your melee weapon attacks have the same size and number of damage dice as the attack that dealt damage, as well as the same damage type.

On your turn, you can use your bonus action to extend the duration of this effect until the end of your next turn.

Skill Absorption

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 short rest to regain all uses of this ability.

Movement. If the creature uses its walking speed to moves a distance greater than your current speed, or uses the dash action, your speed increases to the distance the 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 increases to that number of attacks plus one.

Legendary Resistance. If the creature uses a legendary resistance, you gain proficiency on all saving throws.

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

Cast a Spell. If the creature casts a spell of 3rd level or lower with 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.

Mirror Master

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, as long as you haven't used your reaction, you may immediately strike back with an identical attack. Make an unarmed strike, but on a hit the attack deals the same amount and type of damage as the attack that you just received.