The 'Rex tremendae' begins with a brutal answer to the previous section: A descending arpeggio from the chorus basses in Verdi's best 'maledizione' manner, backed by the lower instruments in unison and a tremolando of upper strings. Tenors divisi repeat the words in a subdued mutter. But the suppliants will not be silenced. The bass launches a contrasting idea ('Salva me fons pietatis'), featuring the rising sixth from soh to mi, whose association with the concept of love would seem to be Mozart's legacy to the Romantic age.

Theme 1 - Rex tremendae -realaudio

Theme 2 - Salva me -realaudio

The two themes engage in conflict. At first the soloists' phrases are isolated each in a different key, linked only by the soprano's cries of 'Salva me'; subsequently they join together, one answering the other. Finally, just when it seems that the battle is lost, a fragmented statement of the bass idea('Salva me fons pietatis') rises like a cloud of incense from the chorus basses to culminate in a cadential variant over wonderfully rich and mellow harmonies.
a note about the 'Maledizione' manner mentioned above:
Verdi had a way of using male choruses enhanced with brass to strengthen certain dramatic moments. The chorus usually expressed wrath at such moments. 'Maledizione' means 'a curse'. One example can be given from Verdi's opera 'La forza del destino'. The chorus invokes a curse on anyone nearing a certain sacred cave.

Maledizione - from 'la forza del destino'.