Adonais is written to commemorate the death of -

Adonais: is a 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.

Percy Bysshe Shelley, (born Aug. 4, 1792, Sussex, England.—died July 8, 1822, at sea off Livorno, Tuscany [Italy]),
His passionate search for personal love and social justice was gradually channeled from overt actions into poems that rank with the greatest in the English language.

Best Works:

• A Defence of Poetry
• A Philosophical View of Reform
• Adonais
• Alastor: or The Spirit of Solitude
• Epipsychidion
• Hymn to Intellectual Beauty
• Letter to Maria Gisborne
• Mont Blanc
• Ode to the West Wind
• Oedipus Tyrannus; or, Swellfoot the Tyrant
• Ozymandias
• Peter Bell the Third
• Prometheus Unbound
• Queen Mab
• Rosalind and Helen
• The Cenci
• The Cloud
• The Masque of Anarchy
• The Necessity of Atheism
• The Revolt of Islam
• The Witch of Atlas
• To a Skylark

Source: Britannica.com