The Hive is new to the Heroes of Might and Magic series and unique to Olden Era. It is an insectoid civilization — though civilization may be the wrong word — emerging out of Jadame's southern volcanic belt, where heat vents and old caldera systems have for millennia supported colonies no surface power thought to count. In the present generation those colonies have begun to coordinate, and to surface, and the rest of Jadame has discovered that the underground does not belong to it after all.
The Hive is not a single mind. It is closer to a federation of broods, each with its own queen-caste matriarch, loosely held together by chemical treaty and shared territory. Broods can and do war with each other, but they will close ranks against any outside power that intrudes into the deep-warm. Their technology is biological: armor is molted, weapons are excreted and hardened, and their siege engines are simply very large insects with a job description.
The Hive does not fight the way other factions fight. It does not have a meaningful concept of negotiated surrender, and it does not seem to value any single unit's survival the way the surface factions do. Its broods can sustain casualty rates that would shatter a human army, and they replenish in days rather than weeks.
For players, the Hive is a swarm faction with surprising depth at the high tiers. Cheap, fast, abundant low tiers; specialized mid tiers; and a queen-caste apex that is genuinely terrifying when allowed to develop.