According to Volo's Guide to Monsters, that's very unlikely. Specifically in the part that talks about the conversion of a Ulitharids into an elder brain:
After the death of the ulitharid's body, mind flayers take its brain
and place it in a brine pool, where it grows into an elder brain over
a few days. This process doesn't work on the brain of an ulitharid
that dies a natural death, as a brain that succumbs to old age is too
decrepit to be used in the creation of an elder brain,
This implies that the brain must be harvested immediately after its death, and that it must be not decrepit. This is somewhat incompatible with lichdom, as the emaciated and dessicated brain of a walking corpse could hardly be described as "not decrepit", and it is hard to create a lich (or variants like the Alhoon) from a brainless body.
Of course, nothing says it is impossible for a living elder brain to become undead.
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
- Cody prepares the spells magic missile and bestow curse after a long rest.
- 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.
- 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.
- 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.
- 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.
- Cody places Hexblade's Curse on Tessa.
You gain a bonus to damage rolls against the cursed target. The bonus equals your proficiency bonus.
- Cody waits for Tessa's flesh to rot.
- 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.
- 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
- Each missile deals 6 extra force damage from Hexblade's Curse
- Each missile deals 5 extra force damage from Empowered Evocation
- There are 11 missiles
- The spell deals 20 extra force damage from Fury of the Small
- 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]
Best Answer
This depicts Orcus raising Cyrog from Out of the Abyss
Warning: this answer is full of spoilers
During the Out of the Abyss adventure, the party travels to a location called Gravenhollow, a library that, among other things, allows the party to experience visions of events of the past, present and future.
The relevant section from Out of the Abyss is "Cyrog Lives! Hail Orcus!" (pg. 158), which details one such vision of Orcus turning up next to the recently deceased Elder Brain "Cyrog" and bringing it back from death, then Cyrog tells the colony that "Orcus has saved Cyrog" and commands them to "follow it [Cyrog] into undeath".
This doesn't actually play much of a part in the adventure, save for providing a plot seed for the DM to run with, but that's what that image at the back of Volo's Guide to Monsters is showing.