Can a 1st level spell from the Aberrant Dragonmark feat be cast solely using pact magic spell slots from a warlock?

Question

For an adventure in D&D 5e starting at level 1, I have created a variant human warlock with the feat Aberrant Dragonmark. I have chosen witch bolt as my first level spell from the feat.

In addition, choose a 1st-level spell from the sorcerer spell list. You learn that spell and can cast it through your mark. Once you cast it, you must finish a short or long rest before you can cast it again through the mark. Constitution is your spellcasting ability for these spells.

My question stems from the fact that the spell comes from the sorcerer spell list and makes me question, would I even be able to cast witch bolt using pact magic spell slots because it can also be found on the warlock spell list? If I can cast it, then my next question is what would be the mark spell's modifier (constitution) or my warlock spell modifier (charisma)?

Answer 1

It's up to the DM to rule on whether or not it counts as a Warlock spell.

It all comes down to one question: does the spell count as a Warlock spell?

If it counts as a warlock spell, then you can cast it with Pact Magic slots, since Aberrant Dragonmark says:

In addition, choose a 1st-level spell from the sorcerer spell list. You learn that spell and can cast it through your mark.

Pact Magic slots can be used to cast Warlock spells that you know. Since you know the spell, if it counts as a warlock spell, you can use Pact Magic. If it doesn't count as a Warlock spell, you can't.

This question addresses this problem: What makes a spell being cast considered to be a {class} spell?

Notably, there are conflicting answers. This breif answer alleges that all we need is for the spell to appear on the Warlock spell list. However, this answer (section 4) alleges that because the Aberrant Dragonmark feat specifies the Sorcerer list, it counts as a Sorcerer spell.

This is not at all clear, so it is up to the DM to make a ruling and stick with it.

If we're counting it as a Warlock spell, use Charisma when casting via Pact slots.

If it counts as a Warlock spell, and we're casting it as a Warlock spell using Pact Magic slots, follow the rules for Pact Magic:

Charisma is your spellcasting ability for your warlock spells, so you use your Charisma whenever a spell refers to your spellcasting ability. In addition, you use your Charisma modifier when setting the saving throw DC for a warlock spell you cast and when making an attack roll with one.

Answer 2

No, Pact Magic spell slots cannot be used beyond casting Warlock spells, and Multiclass spellcasting.

The Warlock Class Feature, Pact Magic says this (emphasis mine):

Pact Magic

Your arcane research and the magic bestowed on you by your patron have given you facility with spells. See Spells Rules for the general rules of spellcasting and the Spells Listing for the warlock spell list.

Spell Slots

The Warlock table shows how many spell slots you have to cast your warlock spells of 1st through 5th level. The table also shows what the level of those slots is; all of your spell slots are the same level. To cast one of your warlock spells of 1st level or higher, you must expend a spell slot. You regain all expended spell slots when you finish a short or long rest.

The Feat Aberrant Dragonmark feat does not say that this spell learned counts as a warlock spell.

What if I'm a Sorcerer? Or a Sor/lock?

It does however count as a Sorcerer spell, in case you wanted to try a similar build with using a Sorcerer instead. Also, if you do become a Sorcerer/Warlock Multiclass, then you have access to this juicy paragraph under the Multiclassing options, (again emphasis mine)

Spellcasting

Your capacity for spellcasting depends partly on your combined levels in all your spellcasting classes and partly on your individual levels in those classes. Once you have the Spellcasting feature from more than one class, use the rules below. If you multiclass but have the Spellcasting feature from only one class, you follow the rules as described in that class.

Pact Magic. If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.

And the Sorcerer Spellcasting feature has the line:

The Sorcerer table shows how many spell slots you have to cast your sorcerer spells of 1st level and higher.

So if you take levels as Sorcerer, you can cast the Aberrant Dragonmark because you learned a Sorcerer spell, and if you Multiclass, Pact Magic slots can be used to cast these Sorcerer spells.

Now, why can we treat the spell learned from the Sorcerer list as a Sorcerer spell?

We can look at the Sage Advice Compendium, which addresses the similar Feat, Magic Initiate.

If you have spell slots, can you use them to cast the 1st-level spell you learn with the Magic Initiate feat? Yes, but only if the class you pick for the feat is one of your classes. For example, if you pick sorcerer and you are a sorcerer, the Spellcasting feature for that class tells you that you can use your spell slots to cast the sorcerer spells you know, so you can use your spell slots to cast the 1st-level sorcerer spell you learn from Magic Initiate. Similarly, if you are a wizard and pick that class for the feat, you learn a 1st-level wizard spell, which you could add to your spellbook and subsequently prepare.

QED.