A work in progress. Not playtested yet, still needs critiquing, but I figured maybe a few people out there could use this as a starting point.
CRANIUM RAT COLLECTIVE TRAITS
Ability Score Increase. Your Wisdom score increases by 2. Your Dexterity score increases by 1.
Age. Your physical bodies live brief lives, but your mind is not bound to the flesh of any one creature.
Alignment. The will of Ilsensine is evil, as are the largest collectives of cranium rats on the planes, being mere manifestations of the illithid god's consciousness. Yet occasionally, by some strange chance, a few rats break away from all that. Whether out of a desire for to redeem yourself and do good, some dark agenda or your own, or base fear of being discovered, you are different from the rest of your kind, an oddity of the planes, and practically unique.
Size. You are composed of a swarm of rats who are individually Tiny. You cannot wear armour or clothing, helmets, gauntlets, cloaks, shields, etc., and you cannot wield weapons. Individual rats may gain the benefit of magical items which are small enough for them to carry, usually rings or amulets, and may use (but not wear) items such as goggles, as well as items which do not need to be carried, like ioun stones.
Speed. Each cranium rat has a base walking speed is 20 feet, and a climb speed of 10 feet.
Darkvision. Each cranium rat can see in dim light within 60 feet of it as if it were bright light, and in darkness as if it were dim light. Cranium rats can't discern colour in darkness, only shades of gray.
Keen Smell. Cranium rats have advantage on Wisdom (Perception) checks that rely on smell.
Hive Mind. Your consciousness arises of a number of cranium rats equal to 3 + your Wisdom modifier + your proficiency bonus in any arcane or psionic spellcasting class. These rats must remain within 5 feet of each other at all times; if the collective's number ever drops below this threshold, you become dazed, and if there are ever fewer than three rats, you immediately lose consciousness, until such time as more rats rejoin your collective. While you are unconscious, any cranium rats act independently, with only animal intelligence.
You can maintain a number of additional rats in your collective equal to your level, provided there isn’t another, larger cranium rat collective present. These are not always the same rats, and they may come and go in regions where cranium rats are plentiful. In such regions, you can add rats to your swarm at a rate of 1d4 per hour; in other regions, you can add rats to your swarm at a rate of 1d4 per week.
Collective. Cranium rats who are not joined into a collective are treated as ordinary rats in all respects (p. 309, PHB), but cranium rats in a collective differ in several ways. Each rat uses your Dexterity, Constitution, and Wisdom scores as well as all your proficiencies, although limited by their Tiny size, limited Strength, and lack of hands. You sense and perceive everything any rat in your collective does. When all the rats in your swarm work in tandem, you can either use their combined ability or your Strength score, whichever is better.
On your turn, you can move, take an action, possibly a bonus action, and a free action like any other character. However, when you move, each rat in your collective can move its speed. When you take your action, you either may cast a spell or command the collective. Rats in your collective can be given the following commands: Attack, Dash, Disengage, Dodge, Help, or manipulate an object; each rat must either take the same action or do nothing. If you do not cast a spell on your turn, you may use a bonus action to command the collective a second time, giving a separate command to those rats who did not act on the first.
Spells and class features affect rats in your collective affect only a single rat at a time, and with the following exceptions.
-Class features which grant an extra attack allows each rat to attack again when you command the collective to Attack, not just one rat.
-The Fighting Styles Archery and Dueling also apply to your ranged and melee spell attacks, respectively.
-The monk’s increased damage die with unarmed strike allows the collective to make a single, concerted strike using your action with the listed damage die; it does not increase the bite damage of each rat in the swarm.
Dispersal. When a rat takes damage, you may subtract that damage from your hit points instead-- if you reduce the damage to 0 hit points, the rat takes no damage. You do not lose consciousness at 0 hit points, but if all rats in your collective ever die, your mind is permanently dispersed, and cannot be reformed by anything short of a wish spell.
Telepathy. You can communicate telepathically with any creature with an intelligence of 3 or higher, provided it is within 10 feet of a rat in your collective.
Languages. You are not physically able to speak or write without magical aid, but you can read and understand Common and one other language of your choice.
Combat Psionics. You know the mage hand cantrip, and one other cantrip of your choice from the wizard or warlock’s spell list. At 3rd level, you can cast spiritual weapon, but once you do so, you must complete a long rest before you can do so again. At 5th level, you can cast mage armor on yourself (-selves) at will, with a reduced duration of 1 minute.
Psychic Feedback. You can take an action to sacrifice any number of rats from your collective. If you do so, you regain an expended spell slot of a level equal to half the number of rats sacrificed. Once you do so, you cannot use this trait again until you complete a long rest.