I have no idea if this is what you are looking for, because your OP appears pretty vague, so I'll toss out an idea in mind anyway.
Edit: It appears you gave your second response while I was typing mine. Do with my post what you will.
I created for a piece of fiction, not involving D&D, a creature called the Spore Queen. I will try to describe her in D&D terms.
In D&D terms, she is an intelligent Ooze-like Aberration. She releases spores and by a complicated process, retrieves DNA of creatures which she adds to her "Library" of creatures. She can then use her body as a womb to create a replicate of that creature, or mix and match characteristics of two or three creatures to make a new monster. Everything she pumps out is loyal to Mother. She communicates telepathically within a range of 100', and is generally stationary, often attached to some large Tree or some creature that she parasitically leeches for nutrition.
She releases spores into the air that travel in the wind, and infect organisms. animals, plants, anything living, risks becoming infected by the spores upon contact, ingestion or inhalation.
-First a Fort save to resist being infected. Once infected, the spores grow and gain a sentience, figure an intelligence score of 4-6ish. If you succeed on the save you continue life as normal.
-If infected, lets say three days later if the disease has not been purged by medicine or magic, the target would then have to make a Will save to fight the intellegence of the disease, succeeding on that save results in resisting possession, losing that save results in death of the mind.
-Finally, if you make the Will safe, make another Fort safe to defeat the disease. Success in this third and final save means you're immune from the spores for the rest of your life, failure means death of your body and you fall down, decompose, and release spores into the air.
If you fail the Will save, you die and become an intelligent undead. In this case, the now undead monster becomes a contagious carrier of the spores, and releases spores into the air and by touch attacks.
This undead will try to infect others for a few days, and then try to return to the Spore Queen, for which it has an instinctive understanding of what direction she is in, but otherwise does not know distance or cannot communicate with her. Within a weeks time, the Undead decays so far that it can no longer move, and it rots, the body decaying into more spores that get released into the air.
These second generation spores land within a warm, moist, dark environment. Given a length of time (Week, month, vary according to DM preference) these spores grow into miniature, mutated versions of the host creature, one size category smaller than the original host. These gremlins run around, eating whatever they can find and wreaking general havoc. Consider them Aberrations. After a week's time, the Spore Gremlins have fed enough and then turn on each other, and by survival of the fittest one, two or three of the brood that grow into adulthood, growing to the size category of the original host. They have the abilities of the original host. They then start to emit spores into the air and by touch attacks and gradually decay. These also try to return to the Spore Queen.
When these Spore Gremlins emit spores, these third generation spores then land on a new host. Same Fort, Will, Fort save. The spores carry the DNA of the first host, merging it into the current host. When this host emits spores, the next generation Gremlins are a blend of traits of the previous hosts. A Bird/Tree, an Elf/Dog, a Shark/Beholder, and so on, and so on. Each generation that has mixed DNA has less Gremlins live due to the mutations, and therefore those that do survive are the most effective mix of the DNA.
The generations rarely make it beyond 3 host DNA mixes, but those that do, are very powerful.
Meanwhile, any spores that carry the DNA of previous hosts that return to the Spore Queen deposit the DNA within her body. She than can intentionally select traits of different creatures and then intentionally create new Aberrations that are her children and defenders. She cannot make Spore versions of these creatures that then decay quickly, these are sterile, stable creatures that live ordinary lifespans, if they didn't fight to the death in defense and at the requests of the Spore Queen Mother.
The Spore Queen's motives are her own. She works to create a small army, collect samples of all living organisms, and store them within her library banks. Perhaps she then leaves to populate entire other worlds. She is not known to ever breed, but it's believed that if she did, it would be by dividing into a separate bud queen, the new queen possessing none of the library of the parent, thus needing to start a new colony.
The Spore Queen herself lacks the ability to move and lacks many offensive capabilities, instead relaying upon her children for her defense. She can be killed with relative ease and would have a very low CR, however her children my be very, very powerful.
Because I've never considered using her in a D&D setting, I don't have stats for her or mechanical details for the spores and Gremlins, but I'm sure I could put together something.