Built with Culture equiv-tech spliced in with 4-dimension teleport shielding, Sphere-Ships are an even 1000 km in diameter and armed and armored to destroy...well, pretty much anything short of an entire star cluster. As with Culture GSVs, they are armored to withstand normal supernovae, armed with gridfire impulsers capable of burning miniature ringworlds from the sky, and a host of secondary weapons both diverse and terrible. In equal parts battleship, carrier, foundry, and mega-city, Sphere-Ships are self-sustaining engines and manufactories of war, carrying hundreds of millions of Servitors and Legion units, as well as hosts of subordinate ships.
Augmented with Bergenholm inertialess drives, Sphere-Ships are capable of almost instantaneously reaching speeds thousands of times in excess of normal Culture drives, and of maneuvering with near-instantaneous acceleration and turning capabilities. Equipped with Dimension Drives, Sphere-Ships are capable of travelling the length and breadth of the Overlord's multi-universal Dominion. The subordinate craft they carry have Bergenholms and realspace and hyperspace drives, but not D-drives.
Sphere-Ships are not as numerous as the dimensions of the Overlord's domain (thousands upon thousands of universes) would suggest... the Overlord has a mild distrust of such concentrations of power being granted independence of movement. Furthermore, as they are designed to allow rapid projection of the Overlord's military and naval power to any designated point, they are not intended to garrison every system, or even every ten-thousandth system. They are front-line shock-troops of war.
(Translation -- you won't see entire fleets made up of hundreds of thousands of these SOBs. They're like Death Stars, not Star Destroyers. But I do have more than one, or two, or a dozen, or a hundred... even if I also have hundreds of places they need to be.)
In addition to normal GSV defenses, a one-hundred-meter thick layer of Vanguard anti-magic alloy sheaths the hull of each sphere, at unimaginable expense (due to the fact that Vanguard alloy cannot be matter-duplicated, but instead requires alchemical forging.) This provides each Sphere-Ship hull with a nigh-total impenetrability to all those forces commonly designated under the group category "magic". Depleted Necronium is a rough analogue to Vanguard Alloy.
Unlike DN, however, Vanguard alloy is also as tensile-resistant as adamantium.
Quite frankly, nothing short of the most huge cosmic powers take on Sphere-Ships in frontal assault.
Known weaknesses (not that there are many)
b) No central computer. There is no hack-one-box-and-control-the-ship computer... ships systems are managed by many "crew unit" AIs working together in network, like a human crew works as a team. While this does grant improved data security (you can't hack a Sphere-Ship from one single point), it also means that the ship cannot simultaneously evaluate all data from everywhere within it.
c) Sensors -- without a Mind to drive them, even Culture sensors are not at their full effectiveness. A ship such as the Andromeda could probably avoid detection by a Sphere-Ship at anything but point-blank range, so long as it avoided high-speed maneuvering and stayed on totally "silent running".
