HD: 3, with CON bonus/penalty to each hit die
AC: base 6 , 3 to 9 [10 to 16] depending on DEX
Attacks 2 arm bites for d4 each, swarm over for 2d4, or 2 weapon attacks; bonus to hit depending on STR
Special defense: Swarm weapon immunity
Size: Small to human
Mind: Low to genius depending on INT
Reaction/Morale modifier: +0/+1
A human-shaped swarm of one-inch-high human-like infinitesimals numbering from 300 (gnome size) to 1800 (majestic human sized). The individual creatures are too small to have much of a functioning brain, but a telepathic link gives them a collective consciousness that lets them perform the incredible acrobatic feat, linked limb to limb, of perambulating and acting en masse in a mockery of humankind.
Its ability scores are each the same and number from 3 to 18, that is, 1 point per 100 individuals, giving the appropriate modifiers as to a character. Its Wisdom bonus determines the keenness of its collective senses, while its Charisma determines its ability to communicate in the common tongue through a kind of shrill, choral piping, using the first person plural.
The lilliviathan attacks by swinging its arms, sometimes leveraging human-sized weapons into its hands whose fingers are the deftest members of the swarm. But when it can, it prefers to leap into close brawling range and swarm over its victim with hundred of tiny cuts and bites, hitting automatically each round (each infinitesimal bearing a pin, shard, or nail as a side arm).
Taking normal damage from fire and other area effect weapons, the lilliviathan is nearly immune to weapon attacks, taking only 1 hp per hit from an edged or piercing weapon, or 1 hp per four points done with a crushing weapon. Each hit point it takes reduces its ability scores by one. If danger looms, the swarm is capable of collapsing, dispersing and oozing through small cracks or tiny passages, though it will thereafter need a few minutes to get back into human shape.
Apparently, individual infinitesimals have gender and sex, pregnant females and rapidly growing infants safeguarded within the "torso." Lilliviathan swarms, if they reproduce at all, do so by mitosis. Food is passed in through the "mouth" and distributed hand-to-hand throughout the corpus.