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While searching for an answer i found this and this question about counterspelling a Mind Flayer. Which does not work because Counterspell states in it's reaction trigger:

when you see a creature within 60 feet casting a spell

and the Mind Flayers Innate Spellcasting tells us:

It can cast the following spells, requiring no components

So we know, Counterspell does not work because a Mind Flayers spellcasting is not observable.

However Mage Slayer reads like this:

When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.

RAW this seems to mean, that a Mind Flayer triggers that melee weapon attack because Mage Slayer does not require you to see the spellcasting. This feels wrong to me because there is no way to even tell the mind flayer casted a spell except the spells effect which might be observable (levitate) or not (detect thoughts).

Is there some kind of ruling, that i am missing?

Thomas Markov
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Crazkur
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2 Answers2

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Yes, the reaction is triggered by the casting of the spell

As you already mention in your question the feat does not require the player to see the spell being cast, and so by the rules, the player is allowed to take the reaction regardless of what type of spell is being cast.

I was not able to find any rulings that would contradict the rules as written in the feat.

I can see why some people would see the ability to detect a spell being cast without any visible components strange but that is after all why the character trained hard to become a mage slayer in the first place.

You could decide that the feat works only on spells with visible casting or effects but that is something best agreed on and house-ruled by the group/DM as it is not mentioned in the rules. For example, Jeremy Crawford, Principal rules designer of Dungeons & Dragons mentioned on twitter that in a similar situation (subtle spell vs. mage slayer) he would rule that

the person with the Mage Slayer feat needs to be aware—by sight or sound—that a spell is being cast

This is not an official ruling and does not effect rules of the game, however it seems like a reasonable solution to the dilemma.

Deeps
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  • @Aaron The character is a fighter who wants to specialize in taking down mages. I will grant him the spidey-sense for magic as a bonus for his training. Therefore i can leave the Mage Slayer feat untouched and use the ruling implied in Crawfords Twitter post linked by Deeps. – Crazkur Sep 08 '20 at 11:27
  • I tried to edit my comment, but accidentally deleted it :c darn mobile – Aaron Sep 08 '20 at 13:18
  • Originally commented that the Mage Slayer feat could be flavored as the person's innate ability to sense ambient pressure changes akin to Weave manipulation/ Psionic abilities. A spidey-sense for "magic," if you will, and that full awareness and understanding of how to utilize this sense could be used for a plot hook – Aaron Sep 08 '20 at 13:20
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Not always, unless you permit metagaming at the table.

Most of the time, the character using Mage Slayer will see the effects of the spell, and this will be adequate for using their reaction granted by the feat.

Obviously, the issue comes down to when the effects of the spell are not obvious. Thankfully, the Player's Handbook and Xanathar's Guide to Everything have some guidance here.

First, from Xanathar's Guide, the section entitled Perceiving a Caster at Work:

If the need for a spell’s components has been removed by a special ability, such as the sorcerer’s Subtle Spell feature or the Innate Spellcasting trait possessed by many creatures, the casting of the spell is imperceptible. If an imperceptible casting produces a perceptible effect, it’s normally impossible to determine who cast the spell in the absence of other evidence.

So in the case of the Mind Flayer, there are no spell components to perceive, so at least the casting of the spell is imperceptible. Next, the Player's Handbok in the section entitled Targets says:

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless a spell says otherwise.

If the spell being cast by the Mind Flayer has an imperceptible effect, taken together with the fact that there are no components required, the casting of the spell and its effects should be entirely unknown to everyone watching. In this situation, your fighter's character should have no idea a spell was cast.

Which brings me to the crux of this answer: metagaming.

Mage Slayer says:

When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.

In the RAWest of RAW interpretations, technically your fighter should be able to use their reaction to make an attack, even if the spell's casting and effects are totally imperceptible. But being able to do this requires relying on player knowledge, knowledge the character should not possess. The character has no idea a spell was cast. The state of affairs, from the perspective of the fighter, is entirely identical, whether or not the Mind Flayer casts this entirely imperceptible spell.

So it comes down to the DM ruling: do you allow the character to take advantage of knowledge that only the player possesses? It's up to you. I would rule that the character cannot use their reaction when the casting and effects of the spell are entirely unknown to the them.

Thomas Markov
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  • To me this brings up the question how does mage slayer work for the PC. Because we might know that the extra attack from mage slayer comes from the enemy casting a spell, but maybe the fighter is thinking he is just super good against the kind of people that cast spells. For some reason he can just hit them better/faster than others. Maybe he doesn't know it is just triggered by them casting a spell and it is only a game mechanic to make the feat balanced. Then I would not consider it metagaming to apply this balancing rule. – findusl Sep 08 '20 at 21:01
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    Also maybe he has some special kind of extra moves that are usually easy deflected, except when a person is concentrating on a spell (afaik a caster has to concentrate the whole casting time. not connected to whether the spell requires concentration for the duration). Then he doesn't have to notice it, he just makes the extra move always, but now it might hit. – findusl Sep 08 '20 at 21:02