Slayer

The Slayer is a fighter subclass that focuses on stances to better situate the fighter into the flow of battle.  Rather than rely on a finite pool of techniques, or just being able to hit harder, or arcane, the Slayer instead uses proper form.   Subscribing to the aspect of 'back to fundamentals', the Slayer is a versatile subclass that is at home using any weapon or fighter fighting style to their advantage.

Proper Form

You've learned the various forms of the Slayer, allowing you to take on stances as you need.  After choosing this at third level, you learn the following stances that you may use.

Battle Wrath: You enter into an aggressive combat stance, opening yourself up to attack, while increasing your own.  While in Battle Wrath form, you gain a +1 to hit and damage using weapon attacks.  This increases to +2 at 10, and +3 at 15.
Drawback: You suffer 1 additional damage from attacks made against you.

Mobile Blade: You enter into a highly mobile stance, quickly moving about the field.  After successfully hitting a creature with a weapon attack, you may move 10 feet as a free action.  This increases to 20 feet at level 10, and your full movement at level 15.  You do not provoke opportunity attacks when moving after successfully attacking.
Drawback: After moving, you suffer a -1 to all weapon attacks to hit.

Iron Defense: You enter into a defensive form, deflecting attacks while showing no openings to your opponent.  While in this form, you gain a +1 bonus to AC.  This increases to +2 at level 10, and +3 at level 15.
Drawback: You suffer a -1 to hit and damage while in this stance.

Duelist's Fury: You enter into a focused form, targeting one enemy to strike and dodge, while ignoring all else.  While in this form, you may mark a creature that you can see within 60 feet of you.  You gain advantage on all attacks against this creature, and they suffer disadvantage on all attacks against you.  At level 10, you may choose two creatures, and at level 3, you may choose 3 creatures.
Drawback: All creatures not marked by you have advantage to attacks against you, and you have disadvantage on any creature not marked by you.

Armor Piercer: You enter into a highly focused state, placing emphasis on the strike to bypass armor to strike true.   While in this form, increase your passive critical chance on weapon attacks by +1 (Critical hit on a 19 and 20, instead of just 20).  Increase this to +2 at level 10, and +3 at level 15.
Drawback: All creatures gain a +1 chance to land critical attacks on you while in this stance.

During your turn, once per turn, you may change your stance as a free action.  Changing your stance again requires a bonus action.  The stance lasts until combat ends, one minute, you end it as a free action, or you are knocked unconscious. 

Power Strike

With extra effort, you are able to strike with extra power.  At the 7th level, after you successfully hit a creature with a weapon attack, you may choose to Power Strike.  Power Strike deals your weapon damage a second time.  You may Power Strike a number of times equal to to your Proficiency Bonus, and you regain all uses after a long rest.

In addition, you gain the following two stances.

Crashing Ram:  When you hit a creature with a weapon attack, they must make a strength saving throw (DC equals to 8 plus you Strength/Dexterity modifier, plus your proficiency bonus).  On a failed save, the target is pushed back 5 feet.  This increases to 10 feet at level 10, and 15 feet at level 15.
Drawback: -1 to Dexterity saving throws.

Grasping Chains: When you hit with a melee weapon attack, you may choose to grapple the creature if it's large or smaller.  DC to escape the grasp is 8+Strength+proficiency bonus.  On a ranged weapon attack, the target must make a Dexterity save (DC is 8+Dexterity+proficiency bonus).  On a failed save, the target's speed is reduced by 15 until the start of your next turn.
Drawback: -1 to atheletics to escape grapples.

Hybrid Form

You are capable of making a hybrid form, using  two of your stances, gaining both strengths and weaknesses.  Upon reaching level 10, when assuming a stance, you can choose stances instead of 1, gaining a hybrid stance.  While in this new stance, you gain the benefits of both stances, although also gain the drawbacks of both.  In addition, you gain the following 2 stances.

Flight of the Dragon: You gain a 10 feet to vertical and horizontal jumping distance.  This increases to 15 feet at level 15.  In addition, you reduce fall damage you take by your Strength or Dexterity plus your Proficiency Bonus.
Drawback: Reduce your AC by -1.

Blood Thirsty Troll: Once per turn, when you make a successful weapon attack against a creature, you regain hit points equal to your Constitution modifier.  At level 15, this increases to your Constitution modifier plus your Proficiency bonus.
Drawback: -1 to Wisdom saving throws.

Slayer's Resolve

Starting at level 15, you have masted a weapon from continued use.  Upon reaching level 15, when you use Power Strike, you gain one of the following effects depending on the damage type of the weapon you're using at the time.

Piercing: When you attack a creature using Piercing damage, you may choose to make an insight check against the target's opposed Deception check.  On a successful insight check, you may lower the target's passive AC against your attack equal to your Strength/Dexterity score.

Slashing:  When you attack a creature using Slashing damage, all creatures within 5 feet of you must make a Dexterity saving throw (DC equals to 8 plus your Strength/Dexterity plus proficiency bonus).  On a failed save, the creature takes your weapon damage.

Bludgeoning: When you attack a creature using Bludgeoning damage, on a successful hit the target must make a Strength saving throw (DC equals to 8 plus your Strength/Dexterity plus proficiency bonus.  On a failed save, the target is knocked prone.

In addition, once per turn, when you change your stance, you gain temporary hit points equal to your Constitution plus Proficiency Bonus.

Relentless Advance

At 18th level, the death of one creature fuels your assault, allowing you to strike again.  At level 18, when you reduce a creature to 0 hit points, you may use your reaction to move up to your speed, and make an attack against a creature of your choosing.  You may use this feature a number of times equal to your proficiency bonus, requiring a long rest to regain all uses.