Tomb World
Tomb Worlds are the worlds which have been inhabited by dormant Necrons for millions of years. Their worlds exist in unknown numbers throughout many parts of the galaxy. A Tomb World is seemingly long abandoned, and many are thought to be the ancient worlds of the Necrontyr, since their architecture shows up on many of the planets. Tomb worlds have a tendency to be barren and lifeless desert worlds.
Many Adeptus Mechanicus sent to these planets for purposes of exploring are often never heard from again, the vile Necron inhabitants not having truly left over the millions of years they had been in stasis.
Known Tomb Worlds include:
Sources
1: Warhammer 40,000 5th Edition Rulebook, p. 180-181
2: Nightbringer by Graham McNeill
3: Codex: Necrons, p. 19
4: Warhammer 40,000: Dawn of War - Dark Crusade (RTS video game by Relic Entertainment)
5: Codex: Necrons, p. 7
6: Warhammer 40,000: Dawn of War - Soulstorm