Briefly, the Defend action lets a player character watch over an ally and be ready to interpose themselves between that ally and a threat. When a characters does so, they become the new target of the incoming attack, and possibly additional attacks that occur afterwards, in place of the original target.
For when things get really desperate, the Give Cover reaction lets a character leap bodily into harm's way to protect a near-dead ally without prior preparation to guard them via the Defend action. Because of this the character giving cover can't take the time or caution to defend themselves against the incoming blow: they protect their ally, but the attack automatically hits them instead.
If the chosen creature or object is targeted by an attack originating from a source that you can see while they are within 10 feet of you, you may spend your reaction to move adjacent to them but must make this choice before the attack roll is made. During your movement you may also exchange places with the defended creature, costing 5 feet of your movement to do so. If the triggering attack is a melee attack you must also end this movement adjacent to the attacker. If the triggering attack is a ranged attack, you must end your movement in a position that partially blocks line of sight to the target.
When you spend your reaction to defend the chosen creature the triggering attack is resolved against your AC instead of the AC of the creature you're defending, and you take any damage caused by the attack in place of its original target. If the triggering attack is a melee attack, is not a critical hit, and you're wielding a melee weapon or a shield or your unarmed attacks deal 1d4 or higher damage, you can make a melee attack roll as part of the same reaction to try and block the attack. If your attack roll is higher than the triggering attack's result, then its damage is halved.
The attack automatically hits you instead of the original target, and you suffer the attack's damage as normal. The attacker still rolls a d20 to see if the attack is a critical hit, but in this case the result is not compared to your AC and a roll of natural 1 is not considered an automatic miss.
The full rules for these two new options are presented below, and a discussion of potential implications for class features with similar effects follows.
Defend
The Defend action is a variation of the ready action, and works in a similar way. However, it has its own rules that handle a specific trigger and the reaction to that trigger. When you take the Defend action, you choose a creature or object within 10 feet of you that you can see.If the chosen creature or object is targeted by an attack originating from a source that you can see while they are within 10 feet of you, you may spend your reaction to move adjacent to them but must make this choice before the attack roll is made. During your movement you may also exchange places with the defended creature, costing 5 feet of your movement to do so. If the triggering attack is a melee attack you must also end this movement adjacent to the attacker. If the triggering attack is a ranged attack, you must end your movement in a position that partially blocks line of sight to the target.
When you spend your reaction to defend the chosen creature the triggering attack is resolved against your AC instead of the AC of the creature you're defending, and you take any damage caused by the attack in place of its original target. If the triggering attack is a melee attack, is not a critical hit, and you're wielding a melee weapon or a shield or your unarmed attacks deal 1d4 or higher damage, you can make a melee attack roll as part of the same reaction to try and block the attack. If your attack roll is higher than the triggering attack's result, then its damage is halved.
Give Cover
As a reaction, you throw yourself bodily in the way of an attack intended for an adjacent creature. You may use this reaction even after the attack is rolled, but you can only give cover against a melee attack if the space you occupy is within the attacker's reach, and you can only give cover against a ranged attack if you partially block the attacker's line of sight to their target. As part of this reaction, you may move 5 feet but can only do so if it moves you into a valid position from which to give cover.The attack automatically hits you instead of the original target, and you suffer the attack's damage as normal. The attacker still rolls a d20 to see if the attack is a critical hit, but in this case the result is not compared to your AC and a roll of natural 1 is not considered an automatic miss.
Implications
The Defend action and Give Cover reaction presented above have some conceptual and mechanical overlap with a couple of existing features, discussed below:
Protection Fighting Style (Fighter, Paladin)
When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. You must be wielding a shield.
This is very much in the same conceptual space. However, a character with the Protection fighting style will still find it useful compared to the Defend action, since they don't have to spend their action and wait and see what happens in order to meaningfully protect someone. It's also a superior choice for the warrior compared to Give Cover, which would result in them automatically taking damage, but that can still be in the their arsenal should things get desperate and they want a way to shield an ally that's a sure thing.
Still, many people already consider the Protection style to be one of the weaker fighting style options, and it might be considered devalued further if alternatives are introduced that any character can use. All things considered, I believe that it would be appropriate to extend the benefits of the Protection style to when the character uses the Defense action, causing the triggering attack to be rolled with disadvantage against the Fighter or Paladin's AC. If you like this change, replace Protection's text with the new version below:
PROTECTION
When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. In addition, when you ready the defend action and use your reaction to take it, the attack roll that triggers your reaction is rolled with disadvantage. You must be wielding a shield to gain either benefit of this feature.
Spirit Shield (Path of the Ancestral Guardian Barbarian)
[...]the guardian spirits that aid you can provide supernatural protection to those you defend. If you are raging and another creature you can see within 30 feet of you takes damage, you can use your reaction to reduce that damage by 2d6.
When you reach certain levels in this class, you can reduce the damage by more: by 3d6 at 10th level and by 4d6 at 14th level.
When you reach certain levels in this class, you can reduce the damage by more: by 3d6 at 10th level and by 4d6 at 14th level.
This occupies a similar role in terms of reducing damage taken by an ally. However, it remains relevant because it doesn't unlike Defend it doesn't spend the barbarian's action, which is better spent on rage-fueled attacks. It also doesn't result in the barbarian taking damage themselves, and it can protect an ally up to 30 feet away. Defend and Give Cover simply give the barbarian new tactical options: when an ally is particularly weak, or being targeted by a very strong enemy, the barbarian might prefer the Defend action as it gives them to redirect damage in its entirety rather than simply reduce it, and soak it using their larger hit point pool and raging damage resistance. Likewise, a barbarian can Give Cover in the knowledge that the damage they suffer will be minimised.
Other Features
Channel Divinity: Rebuke the Violent (Oath of Redemption Paladin) and Opportunist (Way of Shadow Monk) are both examples of reactions that are triggered by an ally being attacked, however as in both other cases these features require only a reaction, and the end result of both features is damage to the attacker rather than defense of the target. They therefore don't directly correlate to Defend or Give Cover and are not particularly impacted by the inclusion of these new options.
