The ankheg is a large insect-like monster with a taste for fresh meat, that spends most of its life burrowing beneath the fertile soil of forests and farmlands. An ankheg has six legs, and most of them have brown carapaces, though some are yellow or yellow and brown in various mottled patterns. Adults are up to 10 feet long and weigh about 800 pounds.

An ankheg burrows using its legs and mandibles. They usually do not leave usable tunnels behind them, the loose dirt collapsing behind them; however, the can construct a tunnel if they wish, burrowing at half speed to do so. They often dig winding tunnels as a kind of lair, which can be 40 feet below the surface.

Groups of ankhegs may occupy the same territory but will not cooperate with each other; the treasure type given should be divided among all the ankhegs in a group.

An ankheg usually lays in wait 5 to 10 feet beneath the surface, until it detects the vibrations of approaching prey with its antennae, when it will burrow up to strike.

Ankheg are gigantic burrowing arthropods with chitinous shells. They can subsist on filtered earth, but are not averse to meat. If necessary an ankheg can spit digestive acid with a range of 30-ft. This uses up its stock of acid, which will not be replenished for half a day, and inflicts 4d8 hp damage to a single target (saving throw vs breath weapons for half damage).

When feeding, an ankheg dissolves its prey before sucking the juices from the shrunken husk, like a spider. This attack inflicts 1d4 hp damage per round.

Ankheg sometimes lurk beneath the earth waiting to feel the vibrations of an approaching creature, thence to attack it by surprise.