A Dolmen Gate is a living stone portal that serves as a gate into a portion of the Webway created by the Old Ones. Dolmen Gate technology was first developed by the Necrons with the aid of the C'tan Nyadra'zath, "the Burning One."
With this technology, the Necrons were able to gain a major advantage during the War in Heaven and were finally able to move rapidly through interstellar space without relying on sublight, anti-matter-powered stasis-ships.
As a species lacking psykers, the Dolmen Gates now represent the only means the Necrons possess for faster-than-light interstellar space travel. By the time of the late 41st Millennium, most of the Dolmen Gates still scattered around the galaxy have been destroyed by the Aeldari or have malfunctioned over the passage of time.
Those few still functioning have allowed certain Necron Dynasties to exert their influence rapidly over a large area. Most notably, the Necrons that arose from the world of Damnos utilised Dolmen Gates to maximum effect and launched raiding parties all over the Ultima Segmentum.
In the closing years of the War in Heaven, one of the primary factors that led to the Necrons' ascendancy was their ability to finally gain access to the Old Ones' Webway. The C'tan known as Nyadra'zath, the Burning One, had long desired to carry his eldritch fires into that space beyond space, and so showed the Necrons how to breach its extradimensional boundaries. Through a series of living stone portals known as the Dolmen Gates, the Necrons were finally able to turn the Old Ones' greatest weapon against them, vastly accelerating the ultimate end of the War in Heaven in a Necron triumph.
The portals offered by the Dolmen Gates are neither so stable, nor so controllable as the naturally occurring entrances to the Webway scattered across the galaxy currently controlled by the Aeldari and their Drukhari cousins.
Indeed, in some curious fashion, the Webway can detect when its environs have been breached by a Dolmen Gate and its arcane mechanisms swiftly attempt to seal off the infected spur from the rest of the Labyrinthine Dimension until the danger to its integrity has passed. Thus, Necrons entering the Webway must reach their intended destination through its shifting extra-dimensional corridors quickly, lest the network itself bring about their destruction.
Of course, in the present age, aeons have passed since the Necrons used the Dolmen Gates to assault their archenemies. The Old Ones are gone, and the Webway itself has become a tangled and broken labyrinth. Many Dolmen Gates were lost or abandoned during the time of the Necrons' Great Sleep, and many more were destroyed by the Aeldari, the Old Ones' successors as the guardians of the Webway.
Those that remain grant access to but a small portion of the immense maze that is the Webway, much of that voluntarily sealed off by the Aeldari to prevent further contamination. Yet the Webway is immeasurably vast, and even these sundered skeins allow the Necrons a mode of travel that far outpaces those of the younger races.
It is well that this is so. As a species bereft of psykers as a result of the loss of their souls during the biotransference process, the Necrons are also incapable of Warp travel, and without access to the Webway, they would be forced to rely once more on slow-voyaging stasis-ships, dooming them to interstellar isolation.
- Codex: Necrons (5th Edition), pp. 8, 27