Quote from marcjohne >>

Man this one gets me thinking... Let's break this down step by step

Step-by-Step Breakdown:

Character level: 11 (spell deals 3d8 damage)
Spellcasting ability modifier: +4
We make a critical hit

🔗Critical Hit

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC. A Critical Hit lets you 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. 

Spell First - Crit Second

You first go through the motion of determening the damage as normal and in the end pick up all the dice, roll them again, and add them together.

Initial roll 3d8: 8, 3, 5. So we add one extra d8 and roll another 8 for yet another d8 and so on. The maximum you could get with a modifeier of +4 is ( 3d8 + 1d8 + 1d8 + 1d8 + 1d8 ) * 2 = 14d8 but you need to be very lucky.

Personal Raging:👍

You have a theoretical maximum of 14d8 but a very low chance of actually getting it. I think you can make a very good statement that this is the result according to RAW. Plus it is very easily explained and executed at the table: "Make everything as the spell describes, then at the end double the dice".

On the other hand, when you get a crit and you have the dice at hand, who does not want to pick up all 6d8 and throw them? 🤔 

Double all Dice as you Roll Them

Let us roll the 6d8 and say we get: 8, 3, 5, 8, 2, 1. For each 8 we can add another d8, In this case we have two of them so we can add 2d8 as per the spells description. Lets see how the critical hit affects this. 

Do what the Spell Describes

For the initial roll double the dice for the crit. But the RAW for the spell is add 1d8 for every 8, ignore the crit and add only 1d8 for every 8 you rolled.

Because our initial roll had two 8 in it we add 2d8 as per the spell description. Lets say the result is 8, 2. We rolled one additional 8 so we add one additional d8.
In this example we rolled for a total of 6d8 + 2d8 + 1d8 = 9d8. The maximum that is possible would be 10d8.

Personal Rating: 🤷‍♂️

This one is wired... your chances of adding additional dice is increased by the inital round of rolling but it's not happening on the added dice? The crit clearly states that you "Roll all the the attack's damage dice twice". But on the other hand, the RAW of the spell also clearly states for each 8 roll another (1)d8. Do with it what you will, it's not my favorite. 

We Double all Dice as we Roll Them ... But What Now?

We had two 8 in the initial roll so the spell tells us to add 2d8, because it is a crit we double them and add 4d8. But What happens if we roll another 8, can we add more dice to the damage roll? There are two answers. 

No you can't: The spell clearly states a maximum of 4d8 extra, tough luck.

Lets take another look at the RAW of the spell: "[...] the maximum number of these d8s you can add to the spell’s damage equals your spellcasting ability modifier." With a Modifier of +4 this means a maximum of 4d8 for a total of 6d8 + 4d8 = 10d8.

Personal Rating: 👎

I simply don't like it. The crit rules allow you to change one part of the spell (your chances of adding additional dice got doubled) but oh no, the other part where you can add a maximum of 4d8 is sacret? Just pick one or the other. If you really think this is correct consider using Interpretation-A instead.

Ps.: I guess the same reasoning that a crit either changes some or no parts of the spell, also applies to the last outcome 🤔 Maybe I should change it from meh to me don't like it.  

Yes you can: The dice added from the crit are seperate from the spell.

Yes the spell states you can add up to a maximum of 4d8, but all of those d8 get doubled as you roll them. So effectively for every 8 we roll we can add 2d8 for a theoretical maximum of 14d8.

Personal Rating: 💥

Not as simple as the first interpretation but has the biggest damage potential. Your maximum is 14d8 with the best chance to actually get it, the players will like it. For every d8 the spell tells you to roll, roll two.  

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tl;dr - In my opinion only two of the four options are viable:

1. You ignore the critical hit and roll every dice as the spell tells you to. Then you note down the damage, pick up all the dice, roll them again and add the two numbers.
2. For every d8 the spell tells you to roll, you roll two. Wenn you can roll an additional 1d8 roll 2d8 but only count it as 1d8 against the maximum.

I would rule the crit is seperate. It specifically states "maximum number of these" (referring to the extra d8 from rolling max) is your modifier.

Whereas crit rules state: " A Critical Hit lets you roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together". It does not state it comes from the same source of pool, and instead implies these additional dice are for all intents and purposes, crit dice.

So it may mind, similar to sneak attack, you roll everything twice. If we ruled this way, you could argue that a rogue only gets 3d8 sneak attack damage,. and a crit doesn't affect that as the rules state they only get 3d8 sneak damage, and therefore can't get more.

Quote from Mithise >>

at level 5 with a +3 spell casting modifier. Does this max at 3d8 or can each d8 explode up to 3d8 making the max at level 5 with a +3 6d8? 

I read it as the first, but it very well could be the later.

"...Make a ranged attack roll against the target. On a hit, the target takes 1d8 damage of a type you choose: ..."

It is not an errata? It is says RANGED ATTACK ROLL, NOT RANGED SPELL ATTACK ROLL??!!

Quote from Lordbat75 >>

"...Make a ranged attack roll against the target. On a hit, the target takes 1d8 damage of a type you choose: ..."

It is not an errata? It is says RANGED ATTACK ROLL, NOT RANGED SPELL ATTACK ROLL??!!

 It was updated after the last errata, but at the time of writing this, it's not in the Changelog - - D&D Beyond

I have been thinking of shenanigans you could pull with this but it would depend on the minutiae of the rules and what your DM allows.

My first thought is Twinned Spell - reading the exact description it says ‘the maximum number of d8s you can add to the spell’s damage equals your spellcasting modifier’ and twinned spell says ‘you make a SINGLE spell target two creatures instead of one’ - since it is one spell being cast, it could be interpreted that between the two targets, you only add up to your spellcasting modifier’s worth in total. However, I’d argue that you could double the amount of dice by rolling damage separately for each target and applying the adding effect to each set of damage.

Then, if you were to multiclass this with Tempest Cleric and used Channel Divinity: Destructive Wrath (if you were using lightning or thunder damage) - you could automatically roll a number of d8s equal to ‘1 + spellcasting modifier’ because with Destructive Wrath ‘When you roll lightning or thunder damage you can use your channel divinity to deal max damage instead of rolling’ meaning you would get an 8 on every die, thus meaning you would automatically add the most d8s you can.

This is deffo something to discuss with your DM to see if they would allow it because it’s not exactly clear by interpretation of the rules but I think it’s pretty cool for a cantrip!

There was a discussion on the D&D Beyond forums a while ago about that second thing, the Destructive Wrath thing. I think the general consensus was that because Destructive Wrath says you deal max damage "instead of rolling", and Sorcerous Burst's exploding die effect says "if you roll an 8", technically this interaction doesn't apply. Some DMs might choose to allow it.