D&D 5E hexblade curse damage bonus with multiple damage types

[Shiroiken]

It doesn’t add dice to the damage roll, though it says,

You place a curse on a creature that you can see within range. Until the spell ends, you deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack.

So, the effect of the spell is to deal 1d6 necrotic damage (a separate damage roll from the 1d8 piercing damage or whatever from the weapon) whenever you hit the target with an attack. The weapon’s damage roll isn’t even mentioned in the effect at all.

The keyword is "extra." Effects that don't require an additional roll, such as a saving throw, are all considered part of the attack's damage. This includes Sneak Attack, Smite, Hex, and Hexblade's Curse. The primary reason this matters is for critical hits, as all damage dice are rolled twice. An attack with a rider (such as a save for extra poison damage) is not doubled on a critical hit, since the trigger for the damage is the saving throw, not the attack.

In the case of Hexblade's Curse, since the damage type isn't given, it must be added to one of the existing damage types. If it was only from weapon attacks, the obvious answer would be to add it to the weapon's damage type, but since it includes spell attacks, it can be of any type. With different damage types on an attack, the player would choose which one is increased. In the case of Booming Blade, however, I would only allow the weapon damage to be increased, as the thunder damage occurs later rather than immediately, simple because of tracking issues (I can't see why anyone would put it on the later damage, but...).

 

[Charlaquin]

The keyword is "extra." Effects that don't require an additional roll, such as a saving throw, are all considered part of the attack's damage. This includes Sneak Attack, Smite, Hex, and Hexblade's Curse. The primary reason this matters is for critical hits, as all damage dice are rolled twice. An attack with a rider (such as a save for extra poison damage) is not doubled on a critical hit, since the trigger for the damage is the saving throw, not the attack.

This is a pretty convincing argument.

 

[auburn2]

In the case of Booming Blade, however, I would only allow the weapon damage to be increased, as the thunder damage occurs later rather than immediately, simple because of tracking issues (I can't see why anyone would put it on the later damage, but...).

At fifth level it deals both weapon damage and thunder damage when you attack.

 

[Shiroiken]

At fifth level it deals both weapon damage and thunder damage when you attack.

Ah! Missed that part, which makes a significant difference. Thunder would probably be the choice, because very few creatures have resistance/immunity to Thunder.