Calculate CR of creatures that share spells (like a hag conven)

Question

I recently got inspired by this stat block, that shows that witches can get more powerful when together. https://www.dndbeyond.com/monsters/green-hag-coven-variant

I want to make a similar creature that gets access to some spells when 3 or more of them are within 5 feet of each other. On their own they are CR 1/2, but together they can suddenly pack a punch. I guesstimated them as CR 1 now, but I wonder if that is correct?

In image form and transcribed:

Polevik field guardian

Small humanoid, neutral

• Armor Class 12 (Leather Armor)
• Hit Points 27 (5d8 + 5)
• Speed 30 ft.

STR	DEX	CON	INT	WIS	CHA
12 (+1)	12 (+1)	13 (+1)	12 (+1)	12 (+1)	12 (+1)

• Skills Stealth +6 (When in grass), Nature +3
• Senses Darkvision 60 ft., passive Perception 10
• Languages Common, Dwarfish
• Challenge 1 (200 XP)

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Shared Spellcasting. While three or more Polevik field guardians are within 5 feet of one another, they can tap into each others inate spellcasting abilities, gaining the ability to cast powerful spells. Their spellcasting ability is Wisdom (spell save DC 11, +3 to hit with spell attacks). The Polevik can innately cast the following spells, requiring no material components but must share the spell slots among themselves.
At will: minor illusion, produce flame, shillelagh
3/day each: burning hands, disguise self, entangle, speak with animals, thunderwave
1/day each: fireball, plant growth, speak with plants.

Nimble escape. The Polevik can take the Disengage or Hide action as a bonus action on each of its turns while in the field.

Actions

Quarterstaff. Melee Weapon Attack: +2 to hit (+5 to hit with shillelagh), reach 5 ft., one target. Hit: 3 (1d6) bludgeoning damage, or 7 (1d8 + 3) bludgeoning damage with shillelagh.

Answer 1

tl;dr: You are correct. It is CR 1.

Disclaimer: I just assume the Shared Spellcasting only works once a day with a specific Polevik field guardian, who can't share spell casting with two different pairs of Polevik field guardians a day.

Side note: Your Stat Block is not RAW conform Hit Dice should be d6 because it is a small creature. Make it 6d6+6, that also is 27 at average.

Challenge Rating is a wild thing, but it is basically calculated with the rules in the DMG on site 274. It basically comes down to five factors: Armor Class, Hit Points, Damage/Round, Attack Bonus and Save DC. The later two are calculated using the proficiency bonus appropriate for the challenge rating and are therefore in a kind of feedback loop.

Considering the table I would have put your original monster at a CR 1/8 at first. Every stat it has fits in that category, it is even underperforming in AC. Nimble escape would maybe put the effective attack roll higher - but that depends if the creature ever gets advantage due to being hidden.

Armor Class and Hit Points don't change because of the new feature, but there is a new kind of attack roll (spell attack) that is higher and a save DC. Now let's calculate the Damage/Round. So in the first round, there is the possibility for a fire ball - which can vary how many creatures are affected, but luckily (but kind of illogically) we don't have to worry about that. A fireball is potential 8d6 fire damage, which is on average 28 fire damage. In the same round the next two creatures could both use Burning Hands: 3d6 means 10.5 fire damage on average for each. So that means, 3 Polevik field guardians could do potential 49 damage in the first round, which is roughly 16 damage for each. On subsequent turns they would do less damage, on the second they could put out another Burning Hand for 10.5 fire damage and twice Shillelagh for 7.5 bludgeoning damage, which is 8.5 for each creature on average.

So considering everything I just mentioned, the average damage for the first two rounds in combat is 74.5 for three creatures, which is somewhere between 12 and 13 damage for each per round (not considering possible opportunity attacks) and puts the creature according to the table in CR 1. Considering, that the other stats didn't change it might be a lower CR, but just because of the fact that the first round can be started with a devastating fireball is enough to justify CR 1.

Please comment if I did something wrong, I don't do this very often.