The 'Dies irae' bursts with a volcanic force, intensified by the tonal non sequitur (A major - G minor). Four tutti thunderclaps, later seperated by powerful blows on the bass drum (its skin tightened so as to give a hard, dry sound), open the piece. The whole section is conceived as an unearthly storm: Rapid scales in contrary motion, peremptory calls to attention on the brass, and a chromatic choral line collapsing into slow triplets - all create the feeling of a catastrophe.
An answering phrase in D minor ('Solvet saeclum in favilla') brings a certain balance with its rythmic symmetry, but it too soon dissolves into new stormy ideas based on the thundering beginning.
A musical motive appearing here for the first time, and sung by the Chorus to the words 'Dies irae, dies illa', shall recur again throughout the 'Dies irae' part, and I shall refer to it from now on as the 'Dies-Irae-theme'. Verdi used this 'Theme' to bind the whole part together, and by the time it appears at the end of the Requiem, it already achieves a certain 'Leitmotive' strength.
More curious, however, is the origin of this noisy-musical-idea. It is, in fact, generated from the soft string motive which hovered above the Chorus in the beginning of the Requiem, when this same chorus which now sings 'Day of wrath', then sung 'Give them eternal peace'..

'Dona eis, domine' -realaudio - This is the theme as appearing in the beginning of the Requiem.

'Dies irae, dies illa' - realaudio - This is the theme as it appears now.

The music subsides into anxious muttering ('Quantus tremor est futurus') and then into silence while the first trumpet call sounds from afar..