Ælfwine (called by the Elves Eriol) is a mariner who appears in the early version of Tolkien's mythos and provides the main storyline for The Book of Lost Tales, as the first Man to find the Straight Road and visit Tol Eressëa after many millennia. His character acts as a catalyst for the telling of the early history of Middle-earth during story-telling sessions in and around the Cottage of Lost Play on the Isle of Tol Eressëa.
Although there is little evidence of this storyline in the published version of The Silmarillion, some of the later writings of Tolkien indicate that he didn't fully abandon the idea. However, although Ælfwine is still referred to in some post-Lord of the Rings writings, Tolkien ultimately changed the intended framework of The Silmarillion from the tale of Ælfwine to one based around Bilbo Baggins' Translations from the Elvish books of "Elvish lore".
Ælfwine is also given as the author of the various translations in Old English that appear in The History of Middle-earth series. A minor discrepancy is that whereas Ælfwine is described as hailing from the north-west of England, his Old English texts are in the Mercian dialect, which was Tolkien's favourite.