The El-Adrel system was an unhabited system that was located close to The Children of Tama space. The fourth planet, El-Adrel IV, was the homeworld of a non-sentient species.
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")