The Best Defense

You gain the Defend Action. When you take the Defend action you defer some or all (but at least one) of your attacks; any attacks not deferred may be used as if you had made the Attack action. As with Dodge, until your next turn attacks made against you are at disadvantage and you make Dexterity saves with advantage. Also until your next turn, you gain a bonus to your AC and Dexterity save rolls equal to the number of attacks you deferred, minimum of one and maximum of your relevant skill bonus (STR for blocking and DEX for parrying). You always get a bonus of at least one by using this feat, even if you have a condition that temporarily impacts your Strength or Dexterity. You lose this benefit if you are incapacitated, unconscious, or for any other reason would be unable to make a melee attack. 

All swords, and Heavy and Two-Handed melee weapons, can be used to block, as can shields. All swords, and Light and Finesse melee weapons, can be used to parry.

You may attempt to Defend with an object that isn't an above weapon. It is up to the DM whether to allow it and what happens. Defending with a training sword or a heavy metal candlestick? Probably allowed! Defending with a piece of paper? Probably not allowed! Defending with a relatively fragile object, like a book or a bow or even a glass bottle? You might not like what happens!

Lots of training in one aspect of combat also applies to another: 

• Fighters using their Action Surge may use both actions to defend, pooling the attacks that two actions would provide. The total bonus still cannot exceed the relevant skill bonus. and being able to use any remaining attacks.
• If you are Alert, you can Defend as a reaction and add an additional +1 up to your relevant skill bonus. You may draw one weapon or shield for free if necessary as part of the Defend reaction.
• If you have the Sentinel feat, when a you would get a reaction melee attack on an enemy attacker to defend a creature as per this feat, you may also add your bonus to the targeted creature's AC.
• Mage Slayer: When you would get a reaction melee attack on the enemy spellcaster as per this feat, you also add your bonus to the spell's target's AC.
• Great Weapon Fighting or Great Weapon Master: you may choose to parry with any weapon with which you can block.
• Two Weapon Fighting or Dual Wielder: if you're duel wielding two weapons with which you can parry, you may use them together to block.

Alright, you useless fools, listen here! Combat is not competitive wood chopping! Clashing swords, a desperate block with a staff, a skilled parry that turns a killing blow into a harmless ricochet, these are the essence of the melee. Of course, any damn good-for-nothing novice can just defend or just attack; I will be teaching you how to do both at once! Reading the flow of the battle will keep you and your fellow soldiers alive, so pay attention!

- An unknown sergeant, haranguing his new levy infantry.