The opera is set in Persia in 480 BC and is very loosely based upon Xerxes I of Persia,

though there is little in either the libretto or music that is relevant to that setting. Xerxes, originally sung by a castrato, is now generally performed as a breeches role; that is, it is a male role played by a mezzo-soprano.

The opening aria, "Ombra mai fA1", sung by Xerxes to a tree (Platanus orientalis), is set to one of Handel's best-known melodies, and is often played in an orchestral arrangement, known as Handel's "largo" (despite being marked "larghetto" in the score).

Ombra Mai fu
Di vegetabile
Cara ed amabile
Soave piu.

Frondi tenere e bell
Del mio Platano amato,
Per voi risplenda il Fato
Tuoni, Lampi, e Procelle
Non vi oltraggino mai la cara pace, Ne giunga a profanarvi Austro rapace.

Translation:
Never was there a shadow
Of Branches Sweeter, more refreshing Or more gentle

Tender and beautiful fronds
Of my beloved plane tree
Let fate smile upon you
May thunder, lightning, and storms Never bother your dear peace Nor may you by blowing winds be profaned