His first sight of the temple was not less successful in deceiving his eye than his first impression of the religion in deluding his mind.
With these hidden and guilty mysteries of the Pagan worship, the vault before which Ulpius now stood with his captives was intimately connected.
The human sacrifices offered among the Romans were of two kinds; those publicly and those privately performed.

The first were of annual recurrence in the early years of the Republic; were prohibited at a later date; were revived by Augustus, who sacrificed his prisoners of war at the altar of Julius Caesar; and were afterwards--though occasionally renewed for particular purposes under some subsequent reigns--wholly abandoned as part of the ceremonies of Paganism during the later periods of the empire.
The sacrifices perpetrated in private were much longer practised.

They were connected with the most secret mysteries of the mythology; were concealed from the supervision of government; and lasted probably until the general extinction of heathen superstition in Italy and the provinces.
Many and various were the receptacles constructed for the private immolation of human victims in different parts of the empire--in its crowded cities as well as in its solitary woods--and among all, one of the most remarkable and the longest preserved was the great cavity pierced in the wall of the temple which Ulpius had chosen for his solitary lurking-place in Rome.
It was not merely as a place of concealment for the act of immolation, and for the corpse of the victim, that the vault had been built.

A sanguinary artifice had complicated the manner of its construction, by placing in the cavity itself the instrument of the sacrifice; by making it, as it were, not merely the receptacle, but the devourer also of its human prey.

At the bottom of the flight of steps leading down into it (the top of which, as we have already observed, was alone visible from the entrance in the temple recess) was fixed the image of a dragon formed in brass.
The body of the monster, protruding opposite the steps almost at a right angle from the wall, was moved in all directions by steel springs, which communicated with one of the lower stairs, and also with a sword placed in the throat of the image to represent the dragon's tongue.