Epic is a long narrative poem, usually on a serious subject, centered about a heroic figure. The earliest epics, known as primary or original epics, were shaped from the legends of an age when a nation was conquering and expanding. Epics date back to prehistoric times. The earliest ones were sung by poets who accompanied themselves on a stringed instrument. These epics had no established text. The singer composed each line as he sang it, following the outline of a traditional tale. Examples of this are the Babylonian epic of "Gilgamesh", of the Greek epic, "The Iliad and the Odyssey" by Homer, and of the Anglo-Saxon, "Beowulf". Literary or secondary epics, written in conscious imitation of earlier forms, are most notably represented by Vergil's "Aeneid" and Milton's "Paradise Lost". The epic, which makes great demands on a poet's knowledge and skill, has been deemed the most ambitious of poetic forms. Some of its conventions, followed by epic writers in varying degrees, include a hero who embodies national, cultural, or religious ideals and upon whose actions depends to some degree the fate of his people. The hero performs great and difficult deeds and is concerned with eternal human problems.