Tomb World

A Tomb World is a planet with a Necron tomb on it (and by "on" we mean "in").

The Necrontyr Empire once spanned the galaxy, but after the Wars of Succession, the disastrous War in Heaven, the biotransference into Necron bodies, and the sharding of the C'tan, the empire was left weak and the galaxy was a mess. The Necrons thus decided to go to sleep and let things settle down, so they sought refuge in great tombs, preparing to wake up in sixty million years' time to retake a healed galaxy.

In that time, the galaxy didn't just stop moving; since the vast majority of the inhabitants of the galaxy (basically everyone but the Eldar, who were too busy having orgies/staying away from Slaanesh to remember them) never heard of the Necrons, they didn't know to check for them when settling planets, and so any inhabited planet (and any uninhabited planet, for that matter) could be a Tomb World. However, as of the 41st Millennium, the sixty million years are up, and the Necrons are starting to wake back up. Most of them are not keen on sharing their world with the upstarts who have moved in during their slumber.

For the unlucky traveler who finds him or herself confined to a tomb world, the list of benefits is a short one, but there are at least a couple. Firstly, the world is unlikely to come under threat by the Tyranids or the forces of Chaos. In the case of the former, the space-vermin consistently take pains to stay clear of Necron domains and especially of any potential engagements, as any prospective biomass to be gained is extremely limited since everything is Blackstone and Necrodermis, whereas combat will only result in a loss as there's simply no out-evolving weaponry that destroys its targets at the atomic level. In the case of the latter, while Chaos has certainly been known to attack Necron holdings, when this happens it is typically with intent to destroy some or all of the world and not of conquest. As with the Tyranids, Chaos will typically bypass Tomb Worlds whenever they can, if for no other reason than, even for Space Marines, you're hard-pressed to find a harder target to hit in 40k than one controlled by the Necrons. Furthermore, Necron technology tends to nullify any Warpfuckery that the slaves of chaos would normally employ. So to warrant an attack, the location itself has to be seen as a severe, operational threat to the larger aims of Chaos, and requiring a planned, dedicated, and highly-coordinated strike to neutralize it, as was the case with Abaddon's targeting of the various Necron pylons around the Cadia system in the lead-up to the 13th Black Crusade.

Notable Tomb Worlds

Kaurava 3. Seems cozy if you're a desert tortoise or a Necron

• Kaurava 3: destroyed by Gorgutz 'Ead 'Unter.
• Kronus: Destroyed by Blud Rehvens.
• Damnos: Setting of the Fall of Damnos. Settled during the Great Crusade and, while rich in resources, was given low priority, Necrons awakened in 973.M41, and one year were controlling the entire planet. The Ultramarines could only gather what remained of the populace and retreat into deep space. The Ultramarines later returned in full Chapter strength and alongside a massive Imperial fleet as well as dozens of Deathwatch kill-teams, utterly rekt the Necrons, destroying their C'tan shard and destroying every single Tomb Complex inside the planet.
• Borsis: An artificial planet-weapon that singlehandedly obliterated two Agri-Worlds and dozens of Imperial Navy and Space Marine ships. It took the sacrifice of the Astral Knights to destroy it by freeing the C'tan at its heart and blowing it up from the inside.
• Pavonis: Setting of the novels Nightbringer and Courage and Honor. Mining World, location of the Tembra Ridge a mountain range over the Nightbringer's tomb, the Ultramarines 4th Company led by Uriel Ventris, managed to stop it from escaping into space, but it was never implied that the Nightbringer shard was ever destroyed.
• Simia Orichalcae: Setting of the novel Caves of Ice. An ice world, uninhabited except for Promethium processing plants scattered across the planet to refine veins of rare ice. It had a Necron Tomb, but it was later destroyed by Ciaphas Cain (though Amberley Vail privately stated that no one was too keen on finding out if they truly were gone.)
• Naogeddon: Setting of the short story Deus ex Mechanicus. It houses a shard of the Deceiver, due to this several Explorator Teams have disappeared while exploring in there.
• Carnac: Former Exodite World, was reclaimed by the Necrons in the Carnac Campaign by Anrakyr the Traveller. During the campaign Anrakyr commissioned aid from Mandragora, Gidrim, Trakonn, Solemnace led by Trazyn the Infinite, and Orikan the Diviner. While the Exodites, got help from Alaitoc, Eldorath Starbane, and Illic Nightspear.In the end the Necrons won, because of suprise reinforcements from the just awakened Necrons of Carnac. The Eldar retreated, and Trazyn got his World Shrine.
• Sarkon: Capital of the Empire of the Severed. When radiation storms wiped the memories of the Necrons in Sarkon, leaving them mindless husks of their former selves, the Master Program's own systems were damaged. The malfunctioning program saw the quiet order it had brought to Sarkon and resolved to carry it to other worlds, adopting the form of the Necron Overlord that it dubbed the "Sarkoni Emperor" to do so. Ever since then it has absorbed four other Tomb Worlds.Other non-Necron worlds were also absorbed, using Mindshackle Scarabs to bring any unruly creatures under its direct control.
• Solemnace: Tomb World of everyone's favourite collector/historian/shameless grave robber. Pretty much just a planet-sized galactic history museum, complete with dioramas and galleries that haven't been touched by living beings since the Horus Heresy.
• Mandragora: Crown World (capital) of the Sautekh Dynasty. All of its Overlords and Lords decided fighting a civil war as soon as they woke up was fun, right up until Imotekh the Stormlord put a stop to that.
• Gidrim: Part of the Sautekh Dynasty, ruled by Nemesor Zahndrekh.
• Pyrrhia: Original Tomb World of Anrakyr the Traveller.
• Morrigar: Former Hive World, gang warfare awakened the Necrons, six months later when Cadian 207th regiment, there was no trace of any inhabitants, human or otherwise. When Anrakyr arrived, the lack of Necrons was blamed was on the Cadian 207th and were wiped out by Anrakyr.
• Hollow Crown: Suhbekhar Dynasty Crown World that is a necropolis forged deep within the center of a star. Its construction was commanded by the Phaeron Ahmontekh due to his fears that his aeons long sleep would make him vulnerable to his enemies.
• Thanatos: Houses the Celestial Orrery, a device capable of forcing stars to undergo a supernova millennia before its time that destroys all the nearby worlds that circle it, unfortunately using it must be done with careful consideration as it could upset the natural order of the cosmos and create a critical chain reaction.
• Avarris: Tomb World of the Sautekh Dynasty, and Imperial Knight world, when the Necrons tried to exterminate the place, they found their gauss technology to be ineffective against the ion shields used by the world's Knights.
• Drazak: Also known as the Bone Kingdom, a world populated almost entirely by Flayed Ones, who are ruled by Valgul the Fallen, a Necron Lord immune to the Flayer virus. Every few months Valgul announces a Time of Bounty, where they launch fleets to raid nearby planets to collect flesh and blood of the living to sate his subject's hunger.
• Silva Tenebris: setting of the Mechanicus game.
• Gladius Prime: setting of the Warhammer 40,000: Gladius - Relics of War.
• A lot of Forge Worlds and Knight Worlds are in fact.