This writhing mass is formed of countless composite creatures that have fused into a single being.
Rat King (CR 3) XP 800
Fused rat swarm
N Medium animal Init +6; Senses low-light vision, scent; Perception +8
--- AC 12, touch 12, flat-footed 10 (+2 Dex) hp 16 (3d8+3) Fort +4, Ref +5, Will +2 Defensive Abilities discorporate, half damage from piercing weapons; Immune critical hits, flanking
--- Speed 15 ft., climb 15 ft., swim 15 ft. Melee melee touch +2 (1d6 plus disease and distraction) Special Attacks disease, distraction (DC 12)
--- Str 10, Dex 15, Con 13, Int 2, Wis 13, Cha 2 Base Atk +2; CMB +2; CMD 14 Feats Improved Initiative, Skill Focus (Perception) Skills Balance +6, Climb +10, Perception +8, Stealth +6, Swim +10; Racial Modifiers uses Dex to modify Climb and Swim
--- Environment any Organization solitary, pack (2–5), or infestation (6–12) Treasure none
--- Discorporate (Ex) A rat king dies when reduced to 0 hit points. When a rat king dies, unless its body has been completely destroyed, its remains immediately become a normal, full-strength rat swarm. Disease (Ex) Filth fever: Melee touch—injury; save Fort DC 12; onset 1d3 days; frequency 1/day; effect 1d3 Dex damage and 1d3 Con damage; cure 2 consecutive saves. The save DC is Constitution-based.
A rat king is formed from numerous rats that have been fused together. Most of these rats are joined at the tail, though others are more completely melded together. Though these component rats may once have been independent creatures, the rat king they have formed is a singular being with a single, rat-like mind.
Siphonophore (CR 7) XP 3,200
Fused jellyfish swarm
N Medium vermin (aquatic) Init +1; Senses darkvision 60 ft.; Perception +0
--- AC 11, touch 11, flat-footed 10 (+1 Dex) hp 54 (12d8) Fort +8, Ref +5, Will +4 Defensive Abilities discorporate, half damage from piercing weapons; Immune critical hits, flanking
--- Speed swim 20 ft. Melee melee touch +9 (3d6 plus distraction and poison) Special Attacks distraction (DC 16), poison
--- Str 10, Dex 13, Con 10, Int —, Wis 10, Cha 2 Base Atk +9; CMB +9; CMD 20 Skills Swim +8, Stealth +27; Racial Modifiers Stealth +16
--- Environment any aquatic Organization solitary or bloom (2–8) Treasure none
--- Discorporate (Ex) A siphonophore dies when reduced to 0 or fewer hit points. When a siphonophore dies, unless its body has been completely destroyed, its remains immediately break apart to form a normal, full-strength jellyfish swarm. Poison (Ex) Melee touch–injury; save Fort DC 16; frequency 1/round for 6 rounds; effect 1d4 Dex; cure 2 consecutive saves.
Found in deep water, siphonophores have bodies that resemble long, winding strands of jellyfish. Ordinary siphonophores are fragile creatures whose component parts die if separated from the main body. The siphonophore described in the stat block above is a monstrous version of the ordinary siphonophore, hardier and more compact than its less-monstrous cousins.
Creating a Fused Swarm
"Fused swarm" is an aquired template that can be added to any swarm of corporeal creatures (hereafter referred to as the base swarm). The base swarm fuses into a single creature that has the statistics of the base swarm, modified as follows.
CR As base swarm +1.
Size and Type The fused swarm is Medium, and loses the swarm subtype.
Defensive Abilities Unless otherwise stated, a fused swarm loses all swarm traits. A fused swarm takes half damage from piercing weapons and gains the following defensive ability. Discorporate (Ex) A fused swarm dies when reduced to 0 or fewer hit points. When a fused swarm dies, unless its body has been completely destroyed, its remains immediately break apart to form a normal, full-strength swarm of the same kind as the base swarm.
Immune A fused swarm is immune to critical hits and flanking.
Attack A fused swarm loses the swarm attack of the base swarm, but gains a melee touch attack that deals the same amount of damage and delivers the same special attacks.
Space and Reach The fused swarm has a space of 5 feet and a reach of 5 feet.
Abilities A fused swarm has a minimum Strength score of 10.
CMB and CMD A fused swarm has a CMB and CMD based upon its ability scores and base attack bonus, as normal.