P.B. Shelly's 'Adonais' is an elegy on the death of-

Adonais, pastoral elegy by Percy Bysshe Shelley, written and published in 1821 to commemorate the death of his friend and fellow poet John Keats earlier that year.
Referring to Adonis, the handsome young man of Greek mythology who was killed by a wild boar, the title was probably taken from Bion’s Lament for Adonis, which Shelley had translated into English.
Written in 55 Spenserian stanzas, Adonais is ranked with John Milton’s “Lycidas” for its purity of classical form.
Source: britannica.com