Let's see... Our Monster...
feeds off of pollution and destruction [...] starts out as a tiny tarry ooze like creature [...] [and] quickly grows into a gargantuan cloud of toxic waste.
Now, this pretty much screams abberation, even better, the description even says clearly it is an ooze for the early shape. And, this might still hold true for the mist/fog of the adult phase, as you'll see in a bit.
Hatchlings (CR 2-4)
One of the better matches for a 'toxic' base ooze living in caustic environments might be the Slime Mold, Mudbog, Mercury Ooze and Gray Ooze at CR2 to CR3 or 4, representing "hatchlings", but other CR2-4 oozes make equally well, if just reskinned a tad and possibly just add strengths in numbers: many many cute oozes!
Young (CR 6)
As they grow, they might 'evolve' into the CR6 Id Ooze variant of the Gray Ooze, or a Roiling Oil, depending on it's diet (flameable vs. caustic stuff). Or we grab a monster that I will explore a bit below: Hungry Fog (link follows in Monster Buffing)
really Adult Monster (CR 10-15 + environment)
When it reaches adulthood, or close to it, it might be better represented by a CR 11 Putrid Ooze, CR13 Bog Scum or CR 15 Riftcreeper, but these are all still "solid" enemies, that might need to be either re-fluffed to represent the mist or they just represent the "herart/core" of the being, and the easily passable surroundings of the caustic fog are then represented by making the whole area filled with some environmental hazard like acid gas or lack of oxygen (suffocation), demanding fort-saves and eating away the armor. This however would clearly increase the challenge by at least one or two ranks, possibly even more, so this sounds more like a really old monster, fitting either as the ultimate boss battle or a group of lvl 10 heroes. But didn't I mention fog & mist earlier? Yes, and here's why:
Monster Buffing of the Hungry Fog (Cr6 + Templates)
There is also an already gaseous ooze: The CR 6 Hungry Fog might represent the being in some state between the CR11+ of the block above (including a toxic environment). However, a 6 seems a bit underwhelmed for a 'young adult' monster, so we better start amping it up. Let's start by adding monster templates, that is after all the easy way: Let's start with Advanced to amp it to CR 7. And even if it is not a real swarm, adding the Apocalypse Swarm template will beef our toxic mist monster up to 10 in a very fitting way, evenm if it is not of swarm subtype (which I personally wouldn't bother adding via the swarm template, too much trouble). Just taking the Apocalypse Swarm, have cutting through it mostly create more monsters, allow it to fix itself and allow split of parts to remerge (which is not stated in either template, but would make sense) as a move action might allow you to tailor the final showdown.
And again, you could add tons of toxic pits to its lair for an increased CR. Or stack another template, just to tip the scales.
The CR 7 Advanced Hungry Fog has roughly the following stats:
- AC 9, 73 HP
- Attack +7 touch (6d6+2 negative energy damage)
- DC10 will save vs. fear (shaken or nothing)
The CR 10 Advanced, Apocalypse Swarm Hungry Fog adds or changes several abilities, most notably:
- charisma based Fear aura in 100 ft (DC 10 will save, remember: that makes two DC10 checks, one for the aura and one for the Bewitching Brume from the base!).
- a nauseating "distraction" effect in it's area (DC 22)
- 126 HP (12d8+12*6); fast healing 10; Spell resistance 17
- AC 11
Even as it is not stated, I would rule that if two pieces of our CR10 Hungry Fog are adjacent, they may merge as a free action
