As a master of combat, you are able to ride the flow and tide of battle with ease. Every time you hit a hostile creature with a weapon attack, you gain 'Stamina' equal to half your proficiency bonus (rounded down). The maximum amount of Stamina you can have at one time is equal to 5 + your Constitution modifier (minimum 4 total Stamina). You can expend stamina to fuel abilities you know. You can learn a number of these abilities equal to half your proficiency bonus (rounded up), and can swap them out for other abilities on this list at the end of a long rest:

• Greatstrike (Costs 6 stamina). You can use this ability when you roll a 1 on the damage dice of a weapon attack to reroll. You must use the new roll.
• Parry (Costs 6-8 stamina). When you are hit by a melee weapon attack, you can use your reaction to gain a bonus to your AC equal to 1 + 1 for each additional point of stamina that you expend when you activate this ability, up to a maximum of +3 to your armor class, against that attack, perpetually causing it to miss. You must see the attack and be wielding a melee weapon to use this ability.
• Commander (Costs 5 stamina). Before you make a Charisma check, you can grant yourself a bonus to it equal to half your proficiency bonus (rounded down) as a reaction.
• Reposition (Costs 4 stamina). As a bonus action on your turn you can use this ability to take the disengage action.
• Challenge (Costs 5 stamina). You call out a warcry as a Bonus Action when you use this ability, drawing attention to yourself. All attack rolls, made by creatures of your choice within 10 feet of you who can hear you, have advantage against you, but disadvantage against all other creatures, until the end of your next turn.

You regain all expended stamina at the end of a short or long rest.