Taken from WD#108 - Terror In The Darkness by Carl Sargent.
The Ambull is a large, barrel-chested creature which adopts an apelike stance. Both arms and legs end in iron-hard claws used for tunnelling through stone. It is an alien, from the distant, hot Death World of Luther MacIntyre IX. It spends most of its time underground, sheltering from the direct heat of the sun. It is a carnivore, usually preying on subterranean creatures. In its own habitat, it lives in family groups, constructing vast tunnel systems of remarkable complexity.
The Ambull attacks with 2 claws and 1 bite. By virtue of its size, it can divide these attacks between two human-size opponents if it wishes, attacking with one claw agains one target and using 2 attacks against the other target.
Psychology: The Ambull causes fear in living creatures under 8 feet tall. A special point is that, because of its stupidity, the Ambull is effectively immune to illusions.
The Ambull has three chief characteristics. First, it is big and vicious. In a confined area, it is a deadly enemy indeed, capable of inflicting lethal damage very quickly. The PCs must try to lure it into an area where several people can get around the creature and attack it, or to get into a position where they can use missile fire and ranged spells while the Ambull cannot get at them. This can only be done with very narrow passages where the Ambull cannot [immediately] follow, but where a line of sight can be established.
The second major characteristic is the stupidity of the beast. It is obvious to you that following prey towards a narrow passage where you cannot follow, but down which they can launch missile attacks, is dumb. It is obvious to you but not to an Ambull. When you run an Ambull, you play to get at the humans and eat them, although you don't stop to eat while any are still attacking you.
The third major characteristic is the ability of the Ambull to tunnel through rock. So powerful are its claws and jaws that it can move through solid rock with an effective movement rate of M2. This allows it to by-pass obstacles and take the most direct route to its prey.