Goo Sacs

A parasitic creature made of goo, it paralyzes its victim with a tendril slid up their behind as a direct means of attaching to and coating the spine. It slurps their brain out the same way and integrates the organ into itself, using it to think with and gaining access to all of its memories. It attaches itself to the victim's genitals and absorbs them, replacing them and taking their form as a kind of disguise, then uses the debrained body to carry itself to more brains, growing its intellect and expanding the physical brain nestled inside itself.

As the creature grows older and eats more brains, it gains an incredible intellect, despite appearances. Being solitary creatures, it does not often travel in groups and only occasionally infects another body. If it encounters a body that is objectively better than the current one it is attached to, it can migrate to the new body and leave the old one behind.