The Under-Empire is the vast, sprawling subterranean domain of the Skaven, an unmapped labyrinth of tunnels, burrows, and buried city-warrens that riddles the earth beneath the entire Old World and far beyond. From the bustling greatest warren of Skavenblight to countless smaller nests, the ratmen have colonised the darkness beneath the feet of surface-dwellers, who mostly deny the very existence of such a race.
This underground realm is a place of teeming millions, choked with the fumes of warpstone furnaces, echoing with the squeal of endless breeding pits, and riven by the perpetual scheming of the great clans. Tunnels stretch beneath mountains, seas, and cities, allowing the Skaven to strike upward without warning and vanish just as swiftly. The Under-Empire is ruled, in name at least, by the Council of Thirteen.
Despite its scale, the Under-Empire is no unified nation but a seething mass of treachery and internecine war, each clan and warlord clawing for supremacy. That the ratmen have not yet overwhelmed the surface owes much to their own boundless capacity for backstabbing.