The (statue of?) the emperor (Commodus) crowns a trophy with (statue of?) a captive beneath. Notice the care taken to render the base on which each statue rests and the care to ensure the emperor dominates the scene through hierarchy of scale.
The figure in the foreground is about to sacrifice the bull/heifer? with a double axe. Notice the emphasis on the detail of the chain holding the animal’s head in place.
The only place I remember seeing the archaeological remains of anchors for such chains is the sanctuary of Apollo Claros in Turkey. At that site the anchors number sufficient for hecatomb. It is unlikely that such large sacrifices were a regular component of worship and thus the anchors by there permanence serve as a memorial to past hecatombs and serve as a promise of more on the same scale.