Monster: The Rat King

See? Must be true - it's a photograph

No one knows how it starts, but every so often two rats become joined at the tail and take on special powers and abilities. Using these, they attract more special rats to join them, increasing their power, and hordes of normal rats to do their bidding. Tales abound of villages and even towns being completely taken over by the multi-bodied super being that results, and of greedy mayors who will offer much to rid themselves of this curse but withhold payment, only to pay a much greater price...

The Start
The first two rats combine to make the smallest Rat King. They can still move about somewhat, at about 6" movement, but the new King will generally prioritise finding a place to hole up in, as by the time the fourth body is added the King will be functionally immobile, barely capable of moving ten yards in a morning.

Each rat body contributes 1hp, but the King lives or dies as a unit. A remove curse will split a King but against this, or similar, magic the king has 5% magic resistance per body. So a king with 12 bodies would have 60% magic resistance, in addition to any applicable saving throw, against any magic which would split the bodies - even a wish. Bodies may be cut off, however, by normal blades.

So long as the bodies are connected and the king as a whole has 1 or more hit points, the king regenerates 1hp per turn and may even feed off itself in times of famine.

The two-body king has a psychic ability of just 10, and psionic defense mode F (Mind Blank). It has the minor psionic discipline of animal telepathy at second level.

The basic Int of a rat king is 1 plus one per body, and a wisdom score one less. Alignment begins as neutral - the king's actions are always to aid ratkind; who or what they harm is of no interest. As Int increases, kings tend towards Neutral Evil, but never very strongly.

Sometimes they take "expansion"

Growth
Assuming that the king is in a locale where more rats are not hard to come by, a new rat will be added to it every week if a roll of 3d6 exceeds the number of bodies currently in the mass, and always on an 18. The maximum number of bodies is two dozen.

Each new body increases psychic ability by 10 (5 attack and 5 defence), adds one attack or defense mode (randomly) and increases the level of performance of psionic disciplines by 1.

New disciplines are gained at the following numbers of bodies:
2: Animal Telepathy
3: Precognition
6: Empathy
8: Domination
10: Clairaudience
12: Clairvoyance
14: Telepathy
16: Hypnosis
18: ESP
20: Suspended Animation (1-5) or expansion (6)
22: Teleportation
24: Shape Alteration (human/humanoid/demihuman, rat, giant rat, or monstrous rat)

Apotheosis
Once 24th level is reached, the Rat King is effectively free to roam the world bringing weal to ratkind and woe to its enemies and is known as the Rat Emperor. If encountered in human form, the Emperor will fight with weapons as a 4th level fighter. In other forms it will fight as a 5HD monster (it will only have 24hp, don't forget) whether it appears to be a rat, a "normal" giant rat, or a monstrous rat the size of a grizzly bear. Damage per hit in these latter forms are 1pt, 1d4, or 2d6.

In humanoid form the Emperor has a Dex of 21 (+4 reaction, -5 defensive) and in other forms it will have an AC of 4. It may wear any armour in humanoid form but will favour magical leather or studded leather.

Also, as humanoid, the Emperor has the following thieving abilities: PP 80%, OL 77%, F/R 70%, MS 81%, HS 90%, HN 80%, CW 120%, RL 20%.

Any rat or ratkin within sight of the Emperor will follow its telepathic commands without expenditure of psionic points. The only exception is if two Emperors meet (see below), in which case each must depend on their own loyalists; all other rats will remain hidden for the duration of the combat.

Followers
Each body attracts a score of normal rats, and every 50 rats brings 1d4 giant rats, so that an 8-body King will have 160 normal rats and 3d4 giant rats which will obey its telepathic commands.

One reaching 14 bodies, it will attract 1d3 were-rats, and another 1d4 at 20 bodies.

Normal rats which are killed will be replaced at a rate of ten per week, assuming that there is some supply, and giant rats at a rate of one per month. Wererat losses will only be replaced by creation of new wererats, or active recruitment by the king, should he be mobile.

Normal and giant rat losses will not normally be replaced during winter months.

Enemies
Both Moondogs and The Cat Lord hate rat kings with a passion and will do whatever is needed to kill one and its followers if found.

Rat kings are unpopular with normal people, of course, as they aim to divert the human food supply to themselves and their followers, but also with adventurers as the controlled rats and their king have no real interest in treasure and do not collect it even in the way that a normal rat might, so any treasure must come in the form of rewards or perhaps some odds and ends scattered about from victims.

However, the greatest enemy of a rat king is another rat king. No two kings will tolerate another within the same general area - village, town, city quarter. Basically, each king claims a territory with a radius of 10 yards per body in the king and disputes caused by overlapping claims must be settled with the death of one or other king.

This reaches its ultimate end point in the Emperor. There can only be one Rat Emperor in the world, and if there are two then they will seek each other out, magically knowing the general direction of the nearest rival.