Is this Eldritch Crystal Familiar overpowered?

Question

My friend is DMing a campaign, and I've been helping him balance out the homebrew content the players request. The most recent request is a custom familiar that the PC is getting through the Magic Initiate Feat to steal the Find Familiar spell.

To support the custom familiar, the player cited Volo's Guide to Monsters with:

Variant: Familiars Any spellcaster that can cast the find familiar spell (such as an apprentice, warlock, or wizard) is likely to have a familiar. The familiar can be one of the creatures described in the spell (see the Player's Handbook) or some other Tiny monster, such as a cranium rat, a crawling claw, a gazer, an imp, a pseudodragon, or a quasit.

Some of the balance difficulty comes from the fact that they want a monster not in the Monster Manual. The player is hell-bent on having a crystal as their familiar, and created this:

Eldritch Crystal
Tiny ooze, chaotic evil
Armor Class 12
Hit Points 3 (1d4)
Speed 10 ft., burrow 5 ft.
STR 1 (−5)
DEX 8 (-1)
CON 12 (+1)
INT 10 (+0)
WIS 3 (-4)
CHA 1 (-5)
Skills: Arcana +2
Senses blindsight 10 ft, passive Perception 6
Languages —
Challenge 0 (10 XP)
Otherwordly Resilience: The Eldritch Crystal takes no damage from damaging spells, and no additional damage from magical weapons.
Innate Spellcasting: The Eldritch Crystal can cast either Blur or Meld into Stone once per short or long rest.
Actions
—

Is it a balanced familiar? If not, what would make it balanced?

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Notes on specific features:

• The Ooze choice (instead of, say, Aberration, Construct, or Fiend) was because of some 3.5e crystalline oozes.
• The magic resistance was specifically asked for because the player is a Forge Cleric who's getting this familiar through a feat. The class gets fire immunity, and the player wanted a familiar who wouldn't be hurt by all the splash and flames.

Answer 1

Agreed that there are too many powers and abilities here, and they're too far out of scope for a familiar. Let's see..

• blindsight: the only base familiar with blindsight is the bat, and his is echolocation. So, not inherently broken, but rare.

• burrow speed: Only familiar with a burrow speed, and tries to make up for it by tanking speed in general. This one is also... a bit odd. Why would an eldritch crystal have a burrow speed? More to the point, it lets you get in and out of certain places rather more easily than you'd otherwise be able to at low level, and gives it a near-guaranteed escape if it's on a surface it can tunnel in. Would be acceptable as a Unique And Shiny Thing, or convert it into a flight speed (for levitation). Possibly remove the ground speed entirely and give it only a flight speed (probably a slightly faster one) to represent the fact that it's a crystal and does not actually have legs. Lets it be a bit weirder (I suspect the player likes weird), but not in a way you need to worry about.

• otherworldly resistance: This is... excessive. Its primary effect is that it lets the player throw around area effects a much as they like without hurting their familiar. It's also not well-justified. In fact, as far as I'm aware, nothing in the game has blanket immunity to spell damage. If he were building a completely different familiar, I might accept immunity to a single element as the one completely awesome thing it had, but that's not what he's building here. Carve it out.

• innate spellcasting: Blur is second level, and increases survivability in a way that other familiars can't match. Meld into Stone is third, and offers both survivability and shenanigans. Both are overpowered, compared to what is expected of familiars. The only other familiar with a spellcasting effect is the octopus, who's water-bound.

• intelligence stat: 10 is way too high for a familiar base. all other base familiars have between 1 and 3.

• constitution stat: higher than any of the base familiars in the book (most of whom have 8). By extension, this is the only familiar with 3 hp. Probably pretty harmless in itself, but underlines the min-maxxing.

In general, then, the familiar as presented has far more survivability than any of the base familiars, as well as being weirdly intelligent.

Suggestion: As a base, keep the blindsight. Convert the burrow and land speeds into a moderate fly speed (levitate). Let him have the con. Drag the base int down to the 1-3 range (possibly with a special feature to let him use the arcana skill with a non-negative modifier). Ditch the otherworldly resistance and the innate spellcasting. At that point, you're a few notches down from a bat (significantly slower flight speed, no land speed, and no attack. The Arcana thing is cute, but not a big deal, and the other advantages are relatively meaningless). You can probably fit in one more interesting thing without breaking the bank. Possible "needs not eat/sleep/breathe" and make his slowish (10 or 15) fly speed also be a swim speed. That lets him explore (slowly) into underwater areas without having to be aquatic, and lets his master cast spells based on noxious fumes at areas that contain him without concern. In the end, it wouldn't be doing anything that other familiars couldn't do, but the combination would be rare and useful, and it would be doing them in a different way, which should appeal to the player involved.

edit: your player has expressed a strong desire for fire immunity. That's a specialized but situationally very powerful effect, which his own tendencies are going to make somewhat less situational. If you want that, it should be pretty costly. Given the character in question, I'd suggest you lose the "needs not eat/breathe/sleep" and swim, lose the blindsight, make the familiar a permanent light source and possibly give it a familiar-esque 1 damage melee attack (fire). Call it a Forge Spark - a sort of least fire elemental associated with the character's god.

Answer 2

RAW

Variant: Familiar on a monster, such as with Imps, Quasits, and Psudeodragons is an NPC only feature.

The familiar variants in the Monster Manual are for monster and NPC spellcasters. PC spellcasters use the PH.

https://twitter.com/JeremyECrawford/status/725190105451888640

The placement of the new text in Volo's is right before listing the Diviner, Enchanter, Evoker, and Illusionist NPC stats. I think it is clear this variant is meant for those spell casters.

That said, what a DM allows, or not, is up to them at the table.

Balance

The warlock gets the option of a boon, which can be an improved familiar. Among those options are: Imp, Psuedodragon, Quasit, and Sprite. The innate spell casting isn't too different from the Imp, Quasit, and Sprite's ability to turn invisible, and the Imp and Quasit's ability to shape change... The resistance to all magical damage would worrying, if it had more than 2 HP.

That said, it is an improved familiar that taking costs them more spells or a magic (summonable) weapon. So, to give it to another spell caster without any tradeoff would seem unfair if you have a Warlock at the table.

I think sharing that resistance would be way overpowered, no matter the class.

Answer 3

Well, one thing is sure. There is just to many different abilities on that creature for a familiar. The amount of uses for it compared to other familiars makes it unbalanced.

Remove the innate spellcasting and make the otherwordly resilience an resistance instead of imunity.