The El-Adrel system was a neutral planetary system that was located halfway between the United Federation of Planets and the Tamarian space. The fourth planet, El-Adrel IV, was the home of a non-sentient species. Prior to 2368, according to Federation records, the entire system was uninhabited.
In 2368, a Tamarian deep space cruiser commanded by Captain Dathon chose this system to attempt communication with the Federation. He chose the fourth planet because the opportunity of working together with a Starfleet captain in defeating a member of this species offered a parallel with a story in their mythology. The USS Enterprise-D was sent to answer their signal. (TNG: "Darmok")