Their tentacles automatically hit and poison? Yikes! Their CR should go up with that.
Edited for clarity, they need just an attack roll, the saving throw was removed.
Cranium Rats (VGtM)
The Secret of NIMH returns to haunt us in the form of Mind Flayer pet rodents, who are created by blasting a normal rat with psionic energies.
A single Cranium Rat is smarter, tougher, and faster than a regular rat. It also has better eyes, the ability to avoid magical detection, and a glowing brain should it need to see something more clearly in the dark. But even accounting for all that, it is a CR 0 creature that is only a CR 0 creature because there is nothing weaker than CR 0. Meaning you are not using Cranium Rats as combat creatures.
These brainy lab rats can function as disposable scouts for their mind flayer masters, or really anyone who manages to get them under their control. The advantages to doing so are the relatively inconspicuous form factor, the immunity to magical detection, and the high intelligence that can easily be linked up to a hivemind. Or even linked up to a daisy-chain of hiveminds leading all the way back to an elder brain (or someone else)
Said hivemind is what you are really using Cranium Rats for. In their swarm form, they gain a massive boost in intelligence, and the ability to control other minds, making them ideal for infiltrating cities on behalf of whomever (Perhaps even themselves, if they are far enough away from any other hivemind). Interestingly enough, the Swarm still has Psionic casting, even though the Alhoon ostensibly does not. A Swarm of Cranium Rats should almost always be aware of an approaching PCs (due to having innumerable spy drone rats), and how it prepares for a confrontation is largely up to how the PCs have been behaving. If the PCs are bent on extermination, traps are a good countermeasure (maybe they dominated a Kobold or some street urchins for manual labor and access to thumbs), and you get to gloat about the irony of the situation. If the PCs are more talkative, encourage it. The Swarm has Telepathy and Comprehend Languages just for this kind of situation. It might be a good idea to have a bunch of psionic sewer rats as your ally instead of whatever else can live down there.
In combat, a Cranium Rat Swarm is going to employ its Psionic
Confusion against a group of targets o
r Dominate Monster on someone who is likely to fail that save, like a Fighter. Afterwards, it will pick a target and start biting for massive damage. Once a swarm hits half HP, it will lose both its psionics and a significant chunk of melee damage. This is a good time for the swarm to consider retreating, as it might not be able to maintain higher brain functions for very long when forced to retreat at low HP.
A swarm that can’t get away as a whole, will try to get at least one rat to escape. Eventually, that rat might become part of a new swarm and share the memories.
Cranium Rats were changed from Beasts to Aberrations in the changeover, a change I do not particularly like. However I do understand that giving Druids a form that could connect to a hive mind would require massive amounts of DM interpretation. Mostly because I had to deal with that in one of my home games. The Swarm more than doubled its HP, and gained a massive amount of psionic damage on their bites.