[RPG] Is this Flying Monkey familiar overpowered

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.

I didn't do any math, just going off of experience building quite a lot of encounters here. If an exact calculation is what you want, leave a comment and I'll delete this answer.

Coinflip encounter

This encounter will come down to the initiatives rolled. With 13 AC and 28 hit points, neither of the swarms should live more than one attacking round from the party at level 6, even taking into account resistances. They should however take up approximately that round of attack by the players.

Which leaves the Living Spellbook, which gets a practical one round of free attacks because the players will be dealing with the swarms. If they're careless or unlucky (aka, you felt like giving them a hard time), the book will use two multiattacks on the sourcerer or artificer, who should go down fast. At that point the combat depends on whether they were able to dispel the mage armor. If the swarms got off an attack before dying, I'd say those chances are low. Otherwise they are high and the fight should be a breeze for the barbarian and fighter (the arcane explosion should do approximately zero agains them with a Strength saving throw).

So the 2 scenarios I see happening are:

Swarms have relative high initiative

• The swarms attack the artificer and sourcerer for about 50% of their HP
• The swarms die
• The Living Spellbook effectively takes out the artificer and sourcerer out of the fight
• Mage armor isn't dispelled, your barbarian and fighter have a hard time, but should barely win out before any deadly casualties. Bad dice rolls for the player result in a loss

Swarms have relative low initiative

• Swarms die immediately
• Sourcerer dispels mage armor on the Living Spellbook
• Living spellbook becomes a glass cannon with 14 AC, which both the barbarian and fighter easily break
• Fight is a breeze, biggest risk is a small paper cut by one of the twirling paper sheets