quattuor hic primum nigrantis terga iuvencos
constituit frontique invergit vina sacerdos,
et summas carpens media inter cornua saetas
ignibus imponit sacris, libamina prima,
voce vocans Hecaten caeloque Ereboque potentem. (243-7)
Q: To whom does sacerdos refer?
Many take it as the Sibyl (cf. 35, 41, 321, 544, 628).
However, as Horsfall notes, the Sibyl told Aeneas above to do such things (duc nigras pecudes; ea prima piacula sunto, 153). Some take it as Aeneas, but he's clearly identified just after (ipse... / Aeneas, 249-50).
Horsfall has an interesting conjecture that it's an anonymous priest. Those who hold the knives and collect the blood are also unnamed (supponunt alii cultros tepidumque cruorem / succipiunt pateris, 248-9).
I think it could definitely just be the Sibyl, with her performing certain rites first before Aeneas does so. But Horsfall's thought is enticing.