How do the damage from the Hexblade’s Curse feature and the Hex and Bestow Curse spells interact/stack with one another?

Question

Hex requires concentration, while bestow curse at a high level and Hexblade’s Curse do not. This allows all three things to stack.

1. Hex deals damage whenever an attack hits
2. Bestow curse deals damage whenever an attack or spell inflicts damage
3. Hexblade’s Curse adds damage to all damage rolls

If all three were applied to a target, for every time I hit it, would it unfold as follows:

Hit, hex damage, bestow curse damage triggered by hit, bestow curse damage triggered by hex, and all 4 damage rolls get boosted by Hexblade’s Curse?

Answer 1

None of the listed effects add an additional damage roll, they all augment the one roll you make.

deal an extra 1d8 necrotic damage (bestow curse, PHB 218)

you deal an extra 1d6 necrotic damage to the target (hex, PHB 251)

You gain a bonus to damage rolls (Hexblade's Curse feature, XGtE 55)

(all emphasis mine)

So while they do stack, the damage would be:

basic damage + d6 (hex) + d8 (bestow curse) + proficiency bonus (hexblade's curse)

Answer 2

The Damage Roll is the result of all the Damage Dice

You only apply your proficiency bonus from the Hexblade Curse once when calculating your damage from an attack.

Damage Rolls vs Damage Dice

The key here is in differentiating Damage Roll (the total for an attack) vs Damage Dice (individual rolls) when determining how to apply these effects.

The following logic then applies, with the requirement in point 2 that bestow curse must be cast at 5th level or higher to avoid this failing due to two concentration mechanics:

1. Hex is active on a target(PHB, 251)

you deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack

2. Bestow Curse is active on a target(fifth level + to remove concentration) (PHB, 218)

your attacks and spells deal an extra 1d8 necrotic damage to the target.

3. Apply Hexblade Curse (XGtE, 55)

You gain a bonus to damage rolls against the cursed target. The bonus equals your proficiency bonus.

4. Your own Weapon (whatever it may be)

That gives you the following sets of damage rolls:

• Weapon Di(c)e

• Hex Damage

• Bestow Curse Damage

The question now is in whether or not these are all separate damage rolls if it is a single damage roll.

Let's look at the Concentration mechanic

We can look at how keeping a concentration up works in conjunction with this.

We know that each separate damage roll from each source of damage (PHB, 203)

You make a separate saving throw for each source of damage

The attack is your source of damage, and all of the bits of that single attack comprise the damage. This would be the same if looking at a standard attack + hunter's mark. You have your total damage which are the damage dice AND the Hunter's Mark die. Only separately rolled attacks, or attacks that are separate like Magic Missile, are considered separate sources of damage.

This is supported by the PHB (196) under Damgage Rolls:

Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target. Magic weapons, special abilities, and other factors can grant a bonus to damage.

Damage Dice

Damage Dice are a separate mechanic as seen by the use of the Fighting Style Great Weapon Fighter which specifically calls out the Dice rather than the roll:

When you roll a 1 or 2 on a damage die...

The language around Hexblade Curse is about Damage Rolls, not damage die. The curse therefore doesn't apply to individual dice or to each separate roll within a damage calculation.

Damage Summary

That leaves you with you the following damage calculation:

Weapon Di(c)e + 1d6(hex necrotic)+1d8(BC necrotic) + Proficiency Bonus(Hexblade Curse)

There is only ONE Proficiency use per roll

Both the PHB (12) and Xanathar's (5) reference the following rule (emphasis mine):

Your proficiency bonus can’t be added to a single die roll or other number more than once.

This further substantiates that on your Damage Roll for the attack, you can't apply your Proficiency to each damage die.

Answer 3

All the extra dice involved are part of the same damage roll.

While there is no direct mention of this for features like Hex and Bestow Curse, critical hits use the same language (emphasis mine):

When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. [THEN] add any relevant modifiers as normal.

This implies that all extra dice are added for the damage roll before any modifiers. This results in the following procedure for your specific case (let's use an unarmed strike for simplicity):

1. Your attack will deal an extra 1d8 from bestow curse $$ 1 + 1d8$$
2. The attack hits and Hex damage is added: $$ 1 + 1d8 + 1d6$$
3. When all damage is rolled, add all modifiers (Base Modifier and Hexblade's Curse): $$ 1 + 1d8 + 1d6 + \text{Proficiency Bonus} + \text{Strength Modifier}$$