If a bonus action attack for Two-Weapon Fighting involves multiple ability modifiers, which ones are added to the damage roll?

Question

This question is entirely inspired by the following:

• Bonus action attack ability score modifier

That question, however, happens to ask about a homebrew method of adding multiple ability modifiers to the bonus action attack of Two-Weapon Fighting and so, ultimately, has a homebrew solution. This question concerns non-homebrew methods of creating this scenario.

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Two-Weapon Fighting states:

[...] You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative. [...]

But what happens when you would add more than one ability modifier to the damage roll? There are a few ways of doing this, and I'll list the ones I know of below:

The Oathbreaker Paladin's Aura of Hate feature:

[...] Starting at 7th level, the paladin, as well any fiends and undead within 10 feet of the paladin, gains a bonus to melee weapon damage rolls equal to the paladin's Charisma modifier [...]

The Ranger's Foe Slayer feature:

[...] Once on each of your turns, you can add your Wisdom modifier to the attack roll or the damage roll of an attack you make against one of your favored enemies. [...]

The Warlock's Lifedrinker Eldritch Invocation:

[...] When you hit a creature with your pact weapon, the creature takes extra necrotic damage equal to your Charisma modifier [...]

Note, that perhaps some of these aren't quite the same as the others, but what I'm asking remains the same: When a feature actually does cause you to add multiple different ability modifiers to an attack made as part of Two-Weapon Fighting which ones are not added to the damage roll?

Answer 1

The text from the Two Weapon Fighting (TWF) bonus attack is referring to a modification of the normal "Attack" action; the only modifier that does not get added is the Str (or Dex, with a Finesse weapon) mod.

All of the other abilities you have mentioned do not limit or restrict when they apply -- so they apply to every attack, regardless of the source.

(And a reminder: This only applies to damage rolls)

Answer 2

You always add other modifiers

The rules for Damage Rolls state:

When attacking with a weapon, you add your ability modifier--the same modifier used for the attack roll--to the damage. A spell tells you which dice to roll for damage and whether to add any modifiers.

This establishes a general rule wherein you add the ability modifier that you used for the attack to your damage.

Two Weapon Fighting creates a specific exception to that rule, however, it is not phrased in a way that indicates a general prohibition on adding an ability modifier to bonus damage. If it were intended to do so then it would be phrased as:

You cannot add any of your ability modifiers to the damage of the bonus attack, unless that modifier is negative.

Since TWF is only a specific exception to the rules on damage rolls for weapon attacks it does not interact with any other bonus damage features.

It would, however, still apply to the weapon damage using unusual abilities such as via Hex Warrior or Shillelagh. This is because such features change the base ability used for the attack. Since WIS/CHA is replacing STR/DEX you only add the ability modifier to the damage because of the general rules for Attack Rolls and Damage Rolls.

We can compare this to the Oathbreaker Paladin's Aura of Hate which grants bonus damage independent¹ of the general rules for Attack Rolls and Damage Rolls.

^{1: Technically the rules for damage rolls state "Magic weapons, special abilities, and other factors can grant a bonus to damage" but this line doesn't connect to the general rule that TWF violates.}