A beautiful Federation colony world overrun by flying neural parasites in Operation -- Annihilate!, where Kirk's brother Sam dies.
Deneva is one of the loveliest colonies in the Federation and a center of interstellar shipping, home to around a million people. Among them are Kirk's brother Sam, his wife Aurelan and their son Peter. In Operation -- Annihilate!, the Enterprise finds the colonists driven to madness by flying, single-celled parasites that attach to their nervous systems and control them through pain.
The creatures have been spreading from system to system along a line toward Deneva. Spock is attacked and suffers terrible pain, and McCoy eventually finds that intense light kills the parasites. The Enterprise sets satellites in orbit to flood the planet with ultraviolet light, freeing the colony.