Illrigger spell - Vengeful Blade - would it break the targets concentration

The spell states:  On a hit, the target suffers the weapon attack’s normal effects and then radiates a dark aura of energy until the start of your next turn. If the target makes an attack or casts a spell before then, the target takes 1d8 necrotic damage and the spell ends.

Would the target need to roll for concentration if they cast a spell? (presuming they are casting a concentration spell).

Rolling for concentration would be moot because the ability says "the target takes 1d8 necrotic damage and the spell ends."

It's an interesting question. I'd go with the rules for simultaneous effects. Assuming the target is casting the spell on their turn, they can choose to take the damage first, thus not having to roll for concentration.

Quote from Davyd >>

Rolling for concentration would be moot because the ability says "the target takes 1d8 necrotic damage and the spell ends."

I think "the spell ends" is talking about the Vengeful Blade spell, not the spell the target is casting. It could be clearer, though.

The target would need to make a concentration check when they take the initial weapon attack, and one when they take the damage which is triggered when casting a spell.

The Vengeful Blade spell is what ends when the target casts a spell, not the spell cast by the target. The wording is the same for a lot of spells whose effects end on a condition.

Quote from wagnarokkr >>

Quote from Davyd >>

Rolling for concentration would be moot because the ability says "the target takes 1d8 necrotic damage and the spell ends."

I think "the spell ends" is talking about the Vengeful Blade spell, not the spell the target is casting. It could be clearer, though.

 Classic MCDM wording, clear as mud.

In that case, the target would need to make a concentration check against the 1d8 necrotic, same as any other damage source

Quote from Davyd >>

Quote from wagnarokkr >>

Quote from Davyd >>

Rolling for concentration would be moot because the ability says "the target takes 1d8 necrotic damage and the spell ends."

I think "the spell ends" is talking about the Vengeful Blade spell, not the spell the target is casting. It could be clearer, though.

 Classic MCDM wording, clear as mud.

In that case, the target would need to make a concentration check against the 1d8 necrotic, same as any other damage source

 Wouldn't it be nice to kill high level spells with cantrip though? Uh maybe too strong huh.

Quote from Frachese >>

It's an interesting question. I'd go with the rules for simultaneous effects. Assuming the target is casting the spell on their turn, they can choose to take the damage first, thus not having to roll for concentration.

 This can't be a real case of simultaneous effects as the casting of the spell has to actually occur to trigger the occurrence of the necrotic damage.

I'd say no because the target takes 1d8 necrotic damage target when it cast a spell, and Concentration occur after the spell is cast and take effect.

Quote from Wtfdndad >>

Quote from Frachese >>

It's an interesting question. I'd go with the rules for simultaneous effects. Assuming the target is casting the spell on their turn, they can choose to take the damage first, thus not having to roll for concentration.

 This can't be a real case of simultaneous effects as the casting of the spell has to actually occur to trigger the occurrence of the necrotic damage.

I agree with this.  The spell is successfully cast and takes effect.  Anything that happens when the spell takes effect successfully happens.  Then, a concentration check is made to maintain concentration thereafter.

Note that for a readied spell or a long casting time spell the concentration check would be made prior to the spell taking effect. 

If you are not required to maintain Concentration while casting a spell, taking damage before you complete the casting has no effect on it though.

So it all depend how the DM interpret the timing of Vengeful Blade to damage the target before or after the spell takes effect.

You do indeed maintain concentration during the casting of a longer casting time spell.  However, I should not have listed this alongside the readied spell example because in this situation the damage occurs after the casting, not during the casting.  So the readied spell is really the only mechanic where there is a delay between the casting of the spell and the release of the spell effect where this damage can become an issue.

Quote from Plaguescarred >>

I'd say no because the target takes 1d8 necrotic damage target when it cast a spell, and Concentration occur after the spell is cast and take effect.

 I think this is the right answer for the OP's question.

You maintain Concentration because it's necessary to sustain the spell's effect after it is cast:

Duration

A spell’s duration is the length of time the spell persists after it is cast. A duration typically takes one of the following forms:

Concentration. A duration that requires Concentration follows the Concentration rules (see the rules glossary). [...]

Concentration

Some spells and other effects require Concentration to remain active, as specified in their descriptions. If the effect’s creator loses Concentration, the effect ends. [...]

Quote from Plaguescarred >>

I'd say no because the target takes 1d8 necrotic damage target when it cast a spell, and Concentration occur after the spell is cast and take effect.

 I don't really understand what you are saying here. 

I think the cantrip will not interrupt anything procedurally and will only trigger off a spell that has fully activated first, then the target of Vengeful blade would indeed roll for Concentration to maintain the spell that triggered the necrotic damage.

The OP's wording makes things difficult, allowing semantical interpretations.  

Quote from markbrisso >>

Would the target need to roll for concentration if they cast a spell? (presuming they are casting a concentration spell).

 If you decide to cast or are casting a spell, no you do not have to roll a concentration check. If you casted a spell, yes you do roll a concentration check to maintain the spell that was successfully and completely casted.

I suppose that also leads to a follow up question, would the necrotic damage, taking affect as the spell is cast, have a chance to disrupt it? I know concentration is covered in the rule book but nothing of disruption while casting (that I know of).  Casting while in under the affects you could focus through cause you're aware of it. But if it happened while casting?

Quote from markbrisso >>

I suppose that also leads to a follow up question, would the necrotic damage, taking affect as the spell is cast, have a chance to disrupt it? I know concentration is covered in the rule book but nothing of disruption while casting (that I know of).  Casting while in under the affects you could focus through cause you're aware of it. But if it happened while casting?

The necrotic damage happens after the spell is cast, not while it's being cast.

Quote from markbrisso >>

I suppose that also leads to a follow up question, would the necrotic damage, taking affect as the spell is cast, have a chance to disrupt it? I know concentration is covered in the rule book but nothing of disruption while casting (that I know of).  Casting while in under the affects you could focus through cause you're aware of it. But if it happened while casting?

No. IIRC there is some feature that specifies that it disrupts a spell bing cast, this one doesn't say that and thus it doesn't do it. In general if you want to disrupt a spell during casting you need Counterspell. 

If you are talking about the caster of Vengeful Blade: no.

If you are talking about the target of Vengeful Blade: Maybe?

• If the spell has a casting time of an action, a bonus action, or a reaction, there is no concentration required to cast the spell and they would only need to make a concentration save if the spell had a duration and required concentration.
• If the spell has a casting time of 1 minute or more, concentration is required during the entire casting time. It is not clear, but the wording of Longer Casting Times suggests that starting, continuing, and completing a casting of a spell all, individually may count as casting a spell and would trigger the spell.

Longer Casting Times

Certain spells—including a spell cast as a Ritual—require more time to cast: minutes or even hours. While you cast a spell with a casting time of 1 minute or more, you must take the Magic action on each of your turns, and you must maintain Concentration (see the Rules Glossary) while you do so. If your Concentration is broken, the spell fails, but you don’t expend a spell slot. To cast the spell again, you must start over.

The underline suggests that the failed, interrupted spell counts a cast spell.

However, the Illrigger is third party content and this is an interaction that is unplanned, at least on WotC's side, and may not have been considered by MCDM Productions. In any case, this is an extreme edge case as probably no one would ever cast a minute+ casting time spell in a situation where they would be targeted by Vengeful Blade.

This cantrip is just booming blade, but with the triggering condition changed from "willingly moves 5 feet or more" to "makes an attack or casts a spell". It would therefore make sense to use the same timing for both. Saying that, I'm not actually sure how it works with booming blade either, does the target take the damage in their original location or the one 5 feet away?

Quote from Wtfdndad >>

Quote from Plaguescarred >>

I'd say no because the target takes 1d8 necrotic damage target when it cast a spell, and Concentration occur after the spell is cast and take effect.

 I don't really understand what you are saying here. 

I think the cantrip will not interrupt anything procedurally and will only trigger off a spell that has fully activated first, then the target of Vengeful blade would indeed roll for Concentration to maintain the spell that triggered the necrotic damage.

 

As written, Vengeful Blade's target radiates a dark aura of energy and If it casts a spell, the target takes 1d8 necrotic damage.

Concentration starts after the spell is cast. Damage is taken when it cast the spell, so to me casting isn't completed when this occur. 

If you interpret it is then it will not interrupt the casting being already completed.