Megalith
Megaliths are Monolith and Doomsday Monolith's father and is thus very large and very powerful. Megaliths are enormous mobile fortresses; in fact, they are so large they can carry multiple Monoliths within its hull, releasing them like a swarm upon the battlefield like a giant Necron Matryoshka Dolls so you can shoot Gauss cannons while you shoot Gauss cannons.
Other than its humongous (humongous what?!) size and capability of carrying as many Monoliths as possible, the Megalith is also equipped with an arsenal of devastating Gauss and particle weaponry far more powerful than your standard-issue Monolith. The Megalith is fully capable of space travel as well as planetary landings, and is used to initiate full-scale planetary assaults against hardened targets, usually from planetary orbit.
A Megalith was first encountered by the forces of the Imperium of Man in the hands of Nemesor Zahndrekh, a Necron Overlord of the Sautekh Dynasty, during the Necrons' Conquest of Uttu Prime in the Vidar Sector in the late 41st Millennium. At Uttu Prime, Zahndrekh's Megalith and its related forces were responsible for eradicating 3 regiments of the Imperial Guard's elite Catachan Jungle Fighters and 3 full companies of Imperial Fists Space Marines. Suffice to say, the Imperials got their asses kicked and were sent home crying that the big meanie Zahndrekh had stolen their lunch money and took them to town for an ass whoopin'. Ever since this incident, the Imperium has stayed the fuck away from Zahndrekh's Megalith.