I was considering adding a second level spell available to artificers, wizards and sorcerers that affected all creatures inside its area (100 feet radius sphere), that only dealt 1d6 damage to each creature on a failed con save. It would be thunder damage and the spellcaster could center the sphere anywhere within 600 feet. 

here is the formatting (the name is lame I admit it, any suggestions would be great)

Baern’s thunderblast

second level evocation

range 600 ft (100ft. radius sphere)

components V, S, M (a sling bullet (20 bullets for 4 cp) which the spell consumes, and a pinch of sulfur)

Used and created by the feared dwarven wizard Baern, this spell was designed to allow casters to truly dominate battlefields where more powerful creatures are less common by wiping out or critically injuring the weaker combatants. Mindful of enemy archers Baern put much work into enhancing the range. When you cast this spell the sling bullet flies out from your grasp and detonates at a point you can see within range. All creatures within 100 feet of the detonation must make a constitution saving throw or take 1d6 thunder damage or half damage on a successful save. 
At higher levels: when this spell is cast at 3rd level or higher, it’s area increases by 10 feet and it’s damage increases by 1d6 for each slot above 2nd.

any feedback would be appreciated. Is it overpowered, underpowered or about right? Am I breaking any taboos which I did not know about? Should I change the die size or damage type?

My thoughts toward balance:

1. It deals a lot less damage than suggested for a second level spell by the damage for homebrew spells table in the DMG, but this is offset by the ridiculous size of the AOE.

2. It has a material cost, even if it is very minor. This makes it somewhat, if not much, harder to spam than its rather low level suggests.

3. Thunder damage is one of the least resisted elemental damage types in the game, meriting a small die size. 

4. Constitution saving throws are generally rather high for more powerful monsters, leading to good chances of saving by the more dangerous enemies, but the spell is designed to be anti-mook, and monsters such as a goblin or guard won’t have amazing saves and are likely to fail.

5. The spell deals half damage on a successful save. Very useful for eliminating even the most lucky commoner

6. the huge AOE will mean that teammates are often caught by the damage as well. This may merit increasing the damage to 1d8 or 1d10, I am not sure.

Beyond a certain size, the area of a spell becomes more conceptual than practical. The average Battle Mat has a 110ft width, (given 5ft per square), so this spell would essentially cover 4 mats.

Most commoner NPCs only have d8 (avg 4) hit points, so if cast in a town, it would reliably kill nearly half of an entire village.
If cast in a densely packed battlefield, it could do up to 1256d6 points of damage (2-dimensional).
If cast in a forest, it might kill all of the wildlife and most of the vegetation in the entire vicinity.

The scaling is way off: A level 4 Fireball only does 12.5% more damage than a 3rd level Fireball. Yours would double the damage (100% increase), and further increase the area by 21%.

So, I would say that this spell is simultaneously a nightmare for a DM, substantially overpowered, and only situationally useful. This is the kind of spell that would work great as a plot device for a villain, but should not be given to players.

You make some good points. I guess the spell is somewhat (a lot) ridiculous when you consider it. I will use it the way you suggest, as a villain plot device, as I can’t find a good way to dial it back. It could definitely be cool for a villain to use, that is for sure. Now I am picturing a villain immune to thunder infiltrating a castle and using something like this to weaken it severely. 
thanks for the feedback! 

I will take your advice.