What is the effect of the “attack's weapon” doing extra damage?

Question

In Xanathar's Guide to Everything (p. 30), the Cavalier Fighter's Unwavering Mark feature says:

In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. You have advantage on the attack roll, and if it hits, the attack's weapon deals extra damage to the target equal to half your fighter level.

What is the meaning of "the attack's weapon deals extra damage"?

Does it mean the same as "you deal extra damage" clauses (so just add damage to that attack's damage roll)? Or does it mean it is a separate instance of damage (roll twice for concentration saves)? Or is it maybe just a typo, and it should read "the weapon attack deals extra damage"? Or does the weapon suddenly start doing more damage to that target?

It seems like very strange wording to me, and I don't recall seeing anything similar.

Answer 1

The reason it says your attack deals weapon extra damage is to try to drive home that it is the same damage as the weapon. Just like adding your strength to the damage of an attack is.

On PHB p. 196, the Damage Rolls section states: "Each weapon, spell, and harmful ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target."

The above shows that your weapon deals a specific damage. You add your fighter level to this damage.

Note that this is only important for resistances, vulnerabilities, and immunities.

Xanathar's attempted to be clearer on effects like this and on the Gloom Stalker rangers additional dice to weapon damage to attempt to avoid the scenarios of confusion that the rogue's Sneak Attack and the Hunter ranger's Colossus Slayer have.

Answer 2

This is only relevant for resistances

Some creatures are immune or resistant to damage from nonmagical weapons.

Damage Immunities Bludgeoning, piercing, and slashing from nonmagical weapons

In the case of Unwavering Mark, the extra damage is keyed to the weapon meaning the damage is modified by such resistance.

Contrast this to other sources of extra damage which are not keyed to the weapon:

Kensei Shot: ... any target you hit with a ranged attack using a kensei weapon takes an extra 1d4 damage of the weapon’s type

One interpretation as a result of this difference in wording is that the extra damage of Kensei Shot may bypass this damage resistance and immunity (those which are specific to weapons) as it is not specifically keyed to the weapon. The Unwavering Mark is likely absent from this interpretation as it specifies the weapon is dealing the damage.