Edmund Spenser is Michael Drayton's authority for the use of the word "legend" in heroic verse.
1748 editor: "As to the Manner of his Poem, it was written originally in Stanzas of seven Lines, which he afterwards changed very judiciously for the Octave, or more musical Stanza of eight Lines. As he followed the Italian Mode in the Structure of his Verse, so he followed it likewise in the Division of his Work, not into Books but into Cantos; being led thereto, as he tells us himself in his learned Preface, by the Example of Edmund Spenser. It is no Wonder therefore, that a Work written with so much Art as well as Truth, and which was equally valuable whether considered as Poem or as a History, was universally admired in an Age when there flourished so many Persons of true Taste and exalted Genius, who knew how to set a just Price upon the Labour both of the Poet and the Historian" Works of Drayton (1748) 5-6.
Oliver Elton: "The most poetical of the Legends (not excluding the later and tamer one on Cromwell, Earl of Essex) is that of Robert, Duke of Normandy, 1596. The story runs obscure and sluggish enough as a canal; but no verse written afterwards in English is so medieval as the preliminary 'flyting' between the two great personifications, Fame and Fortune, who had spread their dark wings over much poetic homilising. Drayton, as this passage alone would prove, had his momentary share of the melancholy of Du Bellay and Spenser, so deep, in spite of being a literary heirloom. Some lines, which follow closely a passage in the House of Fame, are among the latest traced upon the walls of that abode before it came into the hands of Pope, the eminent eighteenth-century restorer" Michael Drayton: a Critical Study (1905) 42-43.
Richard F. Hardin: "The episode in The Faerie Queene that Drayton most often imitates is the marriage of the rivers (IV xi). The Tragicall Legend of Robert, Duke of Normandy (entered for publication in November 1596, ten months after FQ IV-VI) opens with a personification of Isis, Thames, and Medway" Spenser Encyclopedia (1990) 225.
The word LEGEND, so called of the Latine Gerund, Legendum, and signifying, by the Figure Hexoche, things specially worthy to be read, was anciently used in an Ecclesiastical sense, and restrained therein to things written in Prose, touching the Lives of Saints. Master EDMUND SPENSER was the very first among us, who transferred the use of the word LEGEND, from Prose to Verse: nor that unfortunately; the Argument of his Bookes being of a kind of sacred Nature, as comprehending in them things as well Divine as Humane. And surely, that excellent Master, knowing the weight and use of Words, did completely answer the Decorum of a LEGEND, in the qualitie of his Matter, and meant to give it a kind of Consecration in the Title. To particularize the Lawes of this Poeme, were to teach the making of a Poeme; a Worke for a Volume, not an Epistle. But the principall is, that being a Species of an Epick or Heroick Poeme, it eminently describeth the act or acts of some one or other emient Person; not with too much labour, compasse, or extension, but roundly rather, and by way of Briefe, or Compendium. . . .