Make sure if you´re casting this, have the bard play ¨One-Winged Angel¨.

The saving throw DC is always the caster's spell save DC

Spell Save DC = 8 + Proficiency Bonus + Spellcasting Modifier (Spellcasting modifier for Wizard is Inteligence, the Sorceror, Warlock, and Paladin is Charisma, the Cleric, Druid and Ranger is Wisdom.

If Sephiroth was a D&D character...

For reference this spell does 2.86x the recommended damage for a 9th level area of effect spell in the DMG.  It's the fireball of 9th level spells and is over powered for it's level in terms of damage and by a margin much more so than fireball is over powered ( fire ball only does 1.3x the damage of a third level spell). Even if you use this spell on enemies immune to fire damage, the magical bludgeoning alone is still 1.43x more powerful than recommended for a 9th level spell.  It is based on numbers the most over powered spell in game surpassed only by wish which simply has no numbers. 

It's important to know this for a few reasons

1. You should not balance custom spells or effects based on this spell.  This is not the bench mark for a 9th level spell it is well beyond it.
2. You should not swap this spell out for other 9th level spells assuming it is in the same power range, it is allot stronger.  Only very high CR named npcs have this spell and for good reason.

Quote from Mailcs06 >>

An overchanneling evocation Wizard will deal 240 damage with this spell. Dear god.

edit: I now release you can’t overchannel meteor swarm

 You can't as an evocation wizard, but with a bit of multiclassing you can!

Be a level 18 Order of Scribes Wizard, with Prismatic Wall and Meteor Swarm in your spellbook, and be a level 2 Tempest cleric.
Use Meteor Swarm and change it's damage type to lightning with the Order of Scribes feature. Then use your Channel Divinity to deal a total of 240 lightning damage!

And that's against everyone under a single meteor.

Considering Meteor Swarm has 2 damaging types, bludgeoning and fire, I would say that a Scribe Wizard would only be able to switch one of those to lightning. 

Still makes for a sweet 120 lightning damage + 20d6 bludgeoning damage. 

Theoretically prismatic ray can do that but you would have astronomically low odds of dealing this much. I haven’t done the math but I will at some point 

Alas, your example isn't the best, as Fireball also spreads around corners. 

Hypothetically speaking, you can deal 280 minimum damage. As a level 17-18 sorcerer, you can use this spell and transmute spell(that'll be handy) and put your levels into Tempest Cleric. If you have a Grave Cleric Friend, they can make the creature vulnerable to ALL damage for the first attack. So have your grave cleric friend use Channel Divinity, cast this(20d6 Fire+20d6 Bludgeoning) + transmute to lightning(changes fire to lightning damage). Now you use Channel Divinity to get maximum lightning damage. Now you're looking up to 240 Lightning damage + 40 MINIMUM bludgeoning damage for a minimum of 280, an average of around 320, and a maximum of 360 damage, which at maximum is a little over half of a Tarrasque's health. If my math is wrong however, please do correct me, otherwise, Enjoy :)

This specific combo of destruction was inspired by Offbeat_Outlaw, I suggest checking him out for any D&D content.

The quickest way to world domination 

question: when you cast this spell using d&d beyond; there is only an option for the 20d6 fire damage. i understood this spell as casting 20d6 fire damage PLUS 20d6 bludgeoning damage, equaling 40d6 damage total. is this not the case? im wondering if ive been casting this incorrectly...

Quote from TZATheDestroyer >>

question: when you cast this spell using d&d beyond; there is only an option for the 20d6 fire damage. i understood this spell as casting 20d6 fire damage PLUS 20d6 bludgeoning damage, equaling 40d6 damage total. is this not the case? im wondering if ive been casting this incorrectly...

 it does do 40d6 in total. the spell making system for D&D beyond doesn't really handle spells that can do multiple damage types really well. Hence why it only shows 20d6 and not the full 40

You gotta admit that 160d6 dmg total is way too much, as well as the range. 

I swear hit someone on four different parts of his body and use metamagic Quicken Spell + Empower (Maybe)

I dare someone to do that.

Quote from ComeHereYumYum >>

You gotta admit that 160d6 dmg total is way too much, as well as the range. 

I swear hit someone on four different parts of his body and use metamagic Quicken Spell + Empower (Maybe)

I dare someone to do that.

 Did it. Just asked a friend to run a tarrasque encounter for me and.. yeah. It was fun when it killed me 2 rounds after the spell didn’t do enough damage to kill it. Very fun.

1 mile == 5280 ft.

Monk level 7 says no…

I know it’s kinda late 😅, but the DC is dependent on who used the spell. For a player it‘s 8 + Spell-attribute-modifier + proficiency-bonus.

Don't piss off a school of evocation wizard. 

*Evil Wizard casts Meteor Swarm on a seemingly defenseless town*

*Four Sorcerers walk out of a tavern and look up*: Mystra's sake, not this again. *They cast Catapult as an 8th level spell back towards the Evil Wizard and his tower*

*Evil Wizard screams in terror*

*Druid near the Wizard tower mad at Wizard for burning the forest near the tower*