This Feat unlocks Weapon Mastery for the Fighter, Barbarian, Monk, and Blood Hunter.

Weapon Mastery. Your training with weapons allows you to use the Mastery property of two (three for fighters) kinds of Simple or Martial weapons of your choice. Whenever you finish a Long Rest, you can practice weapon drills and change the kinds of weapons you chose. You gain an additional weapon mastery option at levels 4 & 10. 

To apply a Mastery, go to your proficiencies, and add a custom tool proficiency labeled; "WM: List of weapons". 

The default Mastery Properties of all weapons are described below: 

 Greataxe or Halberd. Cleave (Prerequisite: Melee Weapon, Heavy Property): If you hit a creature with a melee attack using this weapon, you can make an attack roll with the weapon against a second creature within 5 feet of the first that is also within your reach. On a hit, the second creature takes the weapon’s damage, but don’t add your ability modifier to that damage, unless that modifier is negative. You can make this extra attack only once per turn.

Quarterstaff, Spear, Longsword, War Pick, or Warhammer. Flex (Prerequisite: Versatile Property): When you hit with a melee attack using this weapon, you deal its Versatile damage even if you’re wielding it with one hand. 

Glaive or Greatsword. Graze (Prerequisite: Melee Weapon, Heavy Property): If your attack roll with this weapon misses a creature, you can deal damage to that creature equal to the ability modifier you used to make the attack roll. This damage is the same type dealt by the weapon, and the damage can’t be increased in any way, other than increasing the ability modifier.

Dagger, Light Hammer, Sickle, Scimitar, or Double-Bladed Scimitar. Nick (Prerequisite: Light Property): When you make the extra attack of the Light property, you can make it as part of the Attack action, instead of as a Bonus Action. You can still make this extra attack only once per turn.

Greatclub, Pike, or Heavy Crossbow. Push (Prerequisite: Heavy, Two-Handed, or Versatile Property): If you hit a creature with this weapon, you can push the creature up to 10 feet away from you if it is no more than one size larger than you.

Mace, Flail, or Morningstar. Sap (Prerequisite: No Other Properties): If you hit a creature with this weapon, that creature has Disadvantage on its next attack roll before the start of your next turn.

Club, Javelin, Light Crossbow, Sling, Whip, Longbow, or Two-Handed Firearm. Slow (Prerequisite: None): If you hit a creature with this weapon and deal damage to the creature, you can reduce its Speed by 10 feet until the start of your next turn. If you hit the creature more than once with this property, the Speed reduction doesn’t exceed 10 feet.

Battleaxe, Lance, Maul, or Trident. Topple (Prerequisite: Heavy, Reach, or Versatile Property): If you hit a creature with this weapon, you can force the creature to make a Constitution saving throw with a DC equal to 8 + your Proficiency Bonus + the ability modifier you used to make the attack roll. On a failed save, the creature has the Prone condition.

Handaxe, Dart, Shortbow, Rapier, Shortsword, Blowgun, Hand Crossbow, or One-Handed Firearm. Vex (Prerequisite: Ammunition, Finesse, or Light Property): If you hit a creature with this weapon and deal damage to the creature, you have Advantage on your next attack roll against that creature before the end of your next turn.