Charon

Charon is a small moon orbiting the dwarf planet Pluto, named after the mythical boatman of the river Styx.

Star Trek

In TOS, the Enterprise encounters natives of the planet Charon in "Let That Be Your Last Battlefield". The ship eventually visits the planet, only to find the entire population dead from racial warfare.

The location of the planet relative to Earth and the rest of the Federation is unspecified.