Large single-celled creatures, all part of one connected organism, that seized control of humanoid hosts through their nervous systems and spread from world to world.
The neural parasites looked like flat, translucent cells about the size of a hand. Each one was part of a larger collective organism. They attacked by stinging a host and wrapping themselves through the victim's nervous system, causing agony until the host obeyed and helped them spread to new worlds.
In Operation -- Annihilate!, the creatures overran the colony on Deneva, killing Kirk's brother Sam. Spock was infected while capturing a specimen. The crew discovered the parasites could be destroyed by intense light, and satellites flooded Deneva with it. Spock was briefly blinded during the test but recovered thanks to a Vulcan inner eyelid.