The Nehdi were a tribe of Payav native to the Nehdi Valley on planet Mestiko. According to their legends, the Nehdi were brought to the valley by the gods shortly after the planet was formed, and that anyone who left the Land would lose their soul. The Nehdi had a centuries-old rivalry with the neighboring Alangabi tribe, whom ancient myths claimed were descended from the soulless ones who had left the valley. Because of the cultural connection to their land, the Nehdi resisted relocation following "the Pulse" in 2265.
In 2293, Nehdi children began dying from a mysterious illness. Dr. Leonard McCoy and Commander Nyota Uhura discovered that it was Alangabi bioweapons, developed in the mid-23rd century, then buried and forgotten, that were seeping into the water supply and causing the deaths. These revelations forced the Alangabi and Nehdi to resolve their rivalry once and for all, with the Nehdi pledging to use their healing expertise to help the Alangabi who had been effected by the Pulse. (TOS eBook: Mere Anarchy: Its Hour Come Round)