Guardian of Hell

The Guardian of Hell (HP: 800), or simply known as Guardian, is the second Boss in Doom 3. It is seen exclusively when the player reaches the end of the Hell level.

The Guardian looks vaguely like a Tyrannosaurus Rex (The designers commented that the Guardian was designed to imply that this was the form of demon Earth's dinosaurs would have feared. Because it is stated the demons once inhabited Earth and that the Guardian was what the dinosaurs feared the most, it could have caused their extinction.), but with larger and more muscular arms ending in large round fists that appear to contain cracks full of lava, muscular legs with hooves, a hunched upper body, a very long tail that ends with a ball with cracks full of lava in it, an armored back with traces of lava, and a head that has stereotypical demonic horns and a pentagram on it. It shares some similarity, and possibly a relation, to the Demon.

Its refuge is a large area of Hell that is filled with large pillars of rock.

Its primary attacks consist of punching with its large fists and pounding the ground, which emits spheres of energy that damage its target.

Seekers
The Guardian of Hell is blind due to it's age: what remains of its eyes are shallow pits.

It summons much smaller creatures dubbed Seekers, two at a time, to "see" for it and locate its targets. The seekers do not attack the player themselves, but alert the Guardian to your location.

Strategy
Killing both Seekers forces the Guardian to generate more, revealing its vulnerable spot: a glowing orb of energy in an opening of the Guardian's back. Severely damaging this orb will kill the Guardian.

After killing the beast, the Soul Cube is released.

Unlike most other demons, the Guardian's body does not dissolve.

Resurrection of Evil
In Resurrection of Evil the head of the Guardian is engraved on some stone tablets. This could mean that either the Guardian left Hell to assist in the invasion, some of Mars' civilization saw it in Hell, or it was simply aesthetically pleasing to the game's designers.