What is the best damage you can do with a 9th level Magic Missile?

Question

So one of this question's To maximize a combo with True Strike, which is the most damaging spell with attack roll? answers got me curious. What is the maximum damage a 20th level character can do with a 9th level Magic Missile by the rules to a single target.

So my best I've been able to find is Aasimar or Goblin WLK 1/WIZ 19

• 9th Level slot (1D4+1)(11) [keep in mind you only roll once for damage not 11 times] per
  How many times do you roll damage for Magic Missile?
• Empowered Evocation (+5)(11) (assuming 20 INT)
• Hexblade Curse (+6)(11) (Proficiency Bonus +6)
• Radiant Soul/Fury of the Small +20 (Level 20 Aasimar/Goblin)

For a total of (D4+12)(11)+20 for (min-max)avg (163-196)179.5

Am I looking at this correctly? Are there other abilities out there that can increase this?

Feel free to include non legendary magic items. NO TOMES PLEASE

Only use things available on D&D Beyond please.

Acceptable books are :

• Basic Rules
• DMG, MM, PHB
• Sword Coast Adventure's Guide
• The Tortle Package
• Volo's Guide to Monsters
• Xanathar's Guide to Everything
• and any officially released Adventure Book

Answer 1

Maximum damage is 428

Here's how (I added some things to make the damage more consistent; average is 418.791):

The Cast

Your Character (let's call him Cody from now on) is a level 20 Goblin (ASs: 10/13/12/18/16/13) with these classes...

• Wizard 10 ---------- [for Empowered Evocation and 2 ASI]
• Cleric/Druid 1 ----- [for Cleric/Druid spell list]
• Sorcerer 4 ----------[for 1 ASI, Extended Spell, and Wild Magic Surge]
• Warlock 1 ---------- [for Hexblade's Curse]
• Bard 2 -------------- [for Jack of all Trades]
• Fighter 2 ----------- [for Action Surge]

... who gets expeditious retreat and shield from his font of magic and carries these items:

• Spellbook containing magic missile and bestow curse
• 6 Spell Scrolls containing contagion (may not need all of them)
• Ioun Stone of Insight (attuned)
• Ioun Stone of Intellect (attuned)
• a wealth of Spell Scrolls containing shapechange

Your Target (let's call her Tessa from now on) is a gas spore because of their exceptionally low Constitution and Wisdom. (although any creature that is not immune to disease or resistant/immune to force, cold, or necrotic is valid if they fail the saves)

The Process

1. Cody prepares the spells magic missile and bestow curse after a long rest.
2. Cody casts shapechange from a scroll with Extended Spell (trying again if he fails the check, 50% chance; attuning to a Luck Blade and/or acquiring the Lucky feat from a special trainer increase the save significantly). This will allow him to transform into a one-eyed shiver (PoTA 207) for the Chilling Mist trait.

If the one-eyed shiver deals damage to a creature in this area, the creature also takes 5 (1d10) cold damage.

3. Cody casts expeditious retreat and shield for 2 hours until he rolls 33 or 34 on the Wild Surge Table (restarting from step 2 and resting as necessary).

33-34 Maximize the damage of the next damaging spell you cast within the next minute.

4. Cody uses a contagion spell scroll (trying again if he fails the check, one from Action Surge; +7 vs 15 yields a probability over rounds 2-7 is 1 - 35%^7 = 99.94%) to cast Contagion (Flesh Rot) on Tessa; she will fail every save.

Flesh Rot: The creature's flesh decays. The creature has disadvantage on Charisma checks and vulnerability to all damage.

5. Cody casts bestow curse (5th level) on Tessa selecting the fourth option; she will fail the save.

While the target is cursed, your attacks and spells deal an extra 1d8 necrotic damage to the target.

6. Cody places Hexblade's Curse on Tessa.

You gain a bonus to damage rolls against the cursed target. The bonus equals your proficiency bonus.

7. Cody waits for Tessa's flesh to rot.
8. Cody casts magic missile with a 9th level slot selecting Tessa as the target for every missile. Tessa takes extra damage from Fury of the Small.

When you damage a creature with an attack or a spell and the creature's size is larger than yours, you can cause the attack or spell to deal extra damage to the creature. The extra damage equals your level.

9. Tessa explodes and 2d4 baby Tessa float slowly toward Cody making him question his decision.

The Calculation

f is force, n is necrotic, c is cold

• Each missile has base damage of 1d4 + 1 force
  • [1d4+1 f]
• Each missile deals 6 extra force damage from Hexblade's Curse
  • [1d4+7 f]
• Each missile deals 5 extra force damage from Empowered Evocation
  • [1d4+12 f]
• There are 11 missiles
  • [11*(1d4+12) f]
• The spell deals 20 extra force damage from Fury of the Small
  • [11*(1d4+12) + 20 f]
• The spell deals 1d8 extra necrotic from bestow curse
  • [11*(1d4+12) + 20 f + 1d8 n]
• Tessa is vulnerable to all damage 99.94% of the time from contagion
  • [1.999*(11*(1d4+12) + 20 f + 1d8 n)]
• The damage is maximized from Wild Magic Surge
  • [1.999*(11*(4+12) + 20 f + 8 n)]
• The creature then takes 1d10 cold from Chilling Mist
  • [1.999*(11*(4+12) + 20 f + 8 n + 1d10 c)]

This makes 391.804 force, 15.992 necrotic, and 10.995 cold damage for a total of 418.791 damage on average [Max: 428]

Answer 2

I can get a little higher, with a Goblin Wiz16/Sorc3/Wlk1. Use Empowered Evocation, Fury of the Small and Hexblade's Curse as before, but also apply the Sorcerer's Empowered Spell metamagic. This lets us reroll our damage die, taking it from an average of 2.5 to an average of ((2.5+2.5+3+4)/4=)3. The spell Bestow Curse also lets us add 1d8 to our damage (sadly only once, but still).

Our range is only a little higher - 164-204 - but our average goes up 10 points to 189.5.

The Unearthed Arcana Lore Wizard would let us change the spell's damage type, which lets us pile a couple more bonuses on, but nothing official currently allows this, so no luck there.