Sector 418-D was a region of space within the Alpha Quadrant. The sector encompassed the Hertex system, including planet Mestiko, as well as the Klingon colony world Tiranax and stretches of both the Klingon-Federation Neutral Zone and the border region between the Klingon Empire and Romulan Star Empire.

On stardate 1015.3, Earth year 2265, pulsar PSR 418-D/1015.3 was first detected in Sector 418-D, and found to be travelling toward the Mestiko system. (TOS - Mere Anarchy eBooks: The Darkness Drops Again, The Blood-Dimmed Tide)