Necros was a Class-M planet orbiting a G-class star, (PROSE: Sanctuary) located in the Twelve Galaxies. Necros was approximately the size of Mars, with three moon. It had an atmosphere much like Earth's, with a winter and summer, and its seas had no salt. (PROSE: Revelation of the Daleks)
Flora and fauna
Blue was the official colour of mourning on Necros, and women's legs were to be covered at all times, as mentioned to Peri Brown by the Sixth Doctor. In addition to Tranquil Repose, Necros was also the location of Kara's food-processing factory, which provided food for starving planets of the Necros's part of the galaxy, as famine was one of the major problems.
Davros, calling himself the "Great Healer," took over Tranquil Repose to create his Imperial Daleks from the Resting Ones, using the rejected people to create a highly concentrated protein and turning them into the food for Kara's factories. Eventually, the Renegade Daleks were summoned and took Davros away for trial. (TV: Revelations of the Daleks)
Behind the scenes
Many years prior to the publication of the official novelisation of Revelation of the Daleks, the New Zealand Doctor Who Fan Club, notorious for such endeavours, released an unofficial novelisation, which, like a licensed one, attempted to reveal more background to the setting than could be established by the TV series. In that version, Necros was alleged to be the 7th planet of the Pherra system, one of the Pherran colonies established "centuries ago" in the early 30th century. This information, however, is not considered valid on this Wiki due to originating in an unofficial source.