Glorfindel first comes in as a Noldor in the account of the escape of Tuor, Idril, Eärendil and many others from the fall of Gondolin in the First Age. When a Balrog waylays the escapees in the Encircling Mountains above Gondolin, Glorfindel battles him on top of a rocky pinnacle. Both fall to their deaths.
Glorfindel reappears at the end of the Third Age as an Elf-lord sent by Elrond to help the wounded Frodo reach Rivendell, as told in The Fellowship of the Ring. In the Peter Jackson film version of the same name, Arwen does the honors instead, a somewhat controversial move (to fans) to bolster the few active female roles.
It is highly unusual (though not impossible) for an Elf to return from the Halls of Mandos, and in the case of Glorfindel it seems simply to have been an accident. In The Return of the Shadow (a volume in The History of Middle-earth), Christopher Tolkien states that some time after the publication of The Lord of the Rings, his father "gave a great deal of thought to the matter of Glorfindel" (p. 214), and decided that it was a "somewhat random use" of a name from The Silmarillion that would probably have been changed, had it been noticed sooner. Tolkien had a well-documented (and confusing!) habit of inventing and changing character names while writing drafts, so this is not too surprising. Nevertheless, seeing that the mistake had been made, Tolkien devised a rather complex story to explain Glorfindel's return, in which Glorfindel was sent back to Middle-earth by the Valar during the Second Age as a kind of predecessor to the Istari
Here is a more in depth look at Glorfindel's Return compliments of = The Lord of the Rings Fanatics Library =
One cannot really get away with simply presenting the story of Glorfindel's return as fact and not discuss the history behind the idea. However, in the end, it seems, there really is not much of a question about the role of Glorfindel's return in Tolkienien canon. To begin, the idea of Glorfindel's Reincarnation is detailed in two essays written in the last years of Tolkien's life. These are published in The History of Middle-Earth Volume 12: The Peoples of Middle-Earth.
One of the first objection's to Glorfindel's return is a questioning of the validity of Elven Reincarnation. However, this is due to a misconception which many people might have (and some who read this essay might not know anything of Glorfindel to begin with. It is easily understandable that not everyone knows everything that this subject covers). Perhaps people have heard it said that Glorfindel would be the only elf to have been reincarnated, and this makes the claim to outrageous, even if Tolkien wrote it. However, it is actually very clearly written that all elves, if they are essentially good, must be reincarnated if they desire to be. This idea appears in Tolkien's writings as early as the late 1950s, at which time it is receives a first full discussion in the text of the "Laws and Customs Among the Eldar," written as part of the second phase of the Later Quenta Silmarillion published in Morgoth's Ring, Volume 10 of The History of Middle-Earth. This places it in an identical setting as those texts which Christopher Tolkien most heavily relied on in the formation of the published Silmarillion. The manner in which reincarnation was further developed, changing from rebirth to reincarnation into an identical body, but already the idea of elves in general being reincarnated is clear. Christopher Tolkien includes the later ideas of reincarnation in "The Athrabeth Finrod ah Andreth" which is published in the same book. In the second essay on Glorfindel in The Peoples of Middle-Earth, Tolkien writes of Elves who had died: "It was therefore the duty of the Valar, by command of the One, to restore them to incarnate life, if they desired it. But this 'restoration' could be delayed by Manwe, if the fëa while alive had done evil deeds and refused to repent of them, or still harboured any malice against any other person among the living." Thus, Glorfindel is not unique in his reincarnation. All good elves, we can presume, were reincarnated, unless, for some reason, they chose not to be (the example of this is Miriel, Finwë's wife and Fëanor's mother). In the published Silmarillion, there is even reference to elven Reincarnation in Finrod Felagund's last words: "I go now to my long rest in the timeless halls beyond the seas and the Mountains of Aman. It will be long ere I am seen among the Noldor again." This is further added to by the description following his passing: "But Finrod walks with Finarfin his father beneath the trees in Eldamar"(The Silmarillion "Of Beren and Lúthien"). Indeed, this is actually quite evident of Reincarnation, as Finrod is said to be alive again with his Father, for Finarfin has never died. And actually, this line dates all the way to back to the Lay of Leithian, composed in 1928 (published in The History of Middle-Earth Volume 3: The Lays of Beleriand), where it has the form: "While Felagund laughs beneath the trees in Valinor, and comes no more to this grey world of tears and war" (Lines 2875-2877) In a revision of the Lay, the word "laughs" is changed to "walks" and Valinor is replaced with Eldamar. Consequently, the idea of Elven Reincarnation is visible in Tolkien's works long before The Lord of the Rings was begun, so it is very possible for Glorfindel to have been a reincarnated Elf. The only thing special about Glorfindel is that he returned to Middle-earth. He is the only reincarnated elf recorded to have done so (excluding Luthien, who returned as a mortal). However, it is only to be expected that Tolkien's story would be full of unique exceptions emphasizing importance and valor.
Objectors also might claim that Glorfindel's reincarnation was only an excuse by Tolkien to cover his use of the same name for two different elves. However, this is only one small reason, though it is indeed a true reason. There are instances of Elves who had the same name. In The Lord of the Rings itself, there are two Rúmils. One, the brother of Haldir of Lórien, and the other is the Noldorin scribe who created the first Tengwar, mentioned in Appendix E. There are also two Gelmirs in The Silmarillion, and there is a Galdor in "The Fall of Gondolin" who shares the name of Galdor of the Havens who appears in "The Council of Elrond." The two names problem can be summarized with the following statement of Tolkien in the second Glorfindel essay: "This repetition of so striking a name, though possible, would not be credible. No other major character in the Elvish legends as reported in The Silmarillion and The Lord of the Rings has a name borne by another Elvish person of importance." Both Glorfindels are extremely important, but at least one Rumil, Gelmir, and Galdor are not. For this reason, Glorfindel's name could not be duplicated, though for these others, it was not as large of problem.
However, one must recognize that this is not the only reason that Tolkien decided the two were one in the same. To begin with, only a few people besides Tolkien ever knew of Glorfindel of Gondolin while Tolkien was still alive, for nothing had been published with him in it. Tolkien easily could have changed the name of the tragic hero of the "Fall of Gondolin." Actually, the name Legolas also appears in the "Fall of Gondolin," but never again appears in writings of the Eldar Days. Apparently, either Tolkien forgot about this Legolas and his small role , or he would have changed his name, though this cannot really be known. What we do know, nevertheless, is that Tolkien was not about to change Glorfindel of Gondolin's name.
But more importantly, the final decision to make the two the same is based largely on the qualities of the two Glorfindels. Tolkien needs for the Glorfindel of Rivendell to have been a Noldo from the Blessed Realm, for Gandalf, discussing Glorfindel, states that "those who have dwelt in the Blessed Realm live at once in both worlds, and against both the Seen and the Unseen they have great power"(The Fellowship of the Ring "Many Meetings"). The only real valid way for an elf of the Blessed Realm to have ended up back in Middle-earth is for him to have been among those of the Noldor who pursued Melkor after he stole the Silmarils. The Glorfindel of Gondolin also really must be Noldorin, for he was highly important figure in the Noldorin city of Gondolin (though there were many Sindar in Gondolin, it doesn't seem likely that one would have achieved Glorfindel's status, and actually, Gondolin was initially said to have been almost entirely Noldorin). One might wonder then, if Glorfindel is such a powerful figure in the Third Age, sent to rescue the Ringbearer and who could scare away even the Lord of the Nazgul, what was he doing in the First Age in Beleriand, since we know also that he was there? A logical explanation would be to connect him to the Glorfindel who does have a part in the Elder Days.
"At any rate what at first sight may seem the simplest solution must be abandoned: sc. that we have merely a reduplication of names, and that Glorfindel of Gondolin and Glorfindel of Rivendell were different persons. . . . Also it may be found that acceptance of the identity of Glorfindel of old and of the Third Age will actually explain what is said of him and improve the story." This statement by Tolkien in the second essay is simply put, very true. Glorfindel of Rivendell needs a history. He is too powerful to not have one, and the shred of information we have in The Lord of the Rings would make him a candidate to be Glorfindel of Gondolin, given that we know Elven Reincarnation to be perfectly valid. Why is Glorfindel so especially powerful? Well, Tolkien explains this by connecting him to Glorfindel of Gondolin. For having sacrificed himself for the sake of the refugees of Gondolin (and thus, Eärendil, the messenger of Men and Elves unto the Valar), he of course would have been granted reincarnation, even though he had followed the rebellious Noldor (and Tolkien here claims, noncontradictingly, that he did not partake in any of the Kinslaying, and followed only out of loyalty to Fingolfin and Turgon). Furthermore, Tolkien can say that he would even be elevated in status: "his spiritual power had been greatly enhanced by his self-sacrifice." The last remaining connection to be made is why is it that Glorfindel, and not a different elf, returned to Middle-earth. Well, this is a bit more difficult to answer. The intitial reason given by Tolkien is that Glorfindel, as seen in his sacrifice, had a deep love for the Elves of Middle-earth, and a desire to continue to aid them to the point of his death and beyond. If an elf were to return to Middle-earth, someone with Glorfindel's attitude would especially be needed (his sacrifice also shows he was willing to endure further pain in Middle-earth, which he would be relieved from by remaining in the Undying Lands). Still, Glorfindel cannot be the only elf like this. But in his sacrifice, he had indeed been raised in power almost to the level of the Maia, another reason. And indeed, Tolkien here states that Glorfindel befriended Olorin (Gandalf) who had a great affection for the Children of Ilúvatar in Middle-earth, and, of course, would eventually come to Middle-earth himself to aid them. Also, the main leaders of the Noldor perhaps would not have been allowed to return, or reincarnated so quickly, for they had led the rebellion and the Kinslaying. This excludes the most famous of the Noldor, who perhaps would not anyway have been likely candidates for Tolkien to place back into the story because of their status. This leaves Glorfindel as a very good candidate out of a select few Noldor who are important in the First Age, but not too important. Indeed, this question I even find trivial, for if Tolkien says it is Glorfindel who returned to aid the Elves of Middle-earth, it seems that it would be prudent to accept the fact, especially when it does indeed "improve the story."
Still, there is a final objection, one which often finds its base in the fact that within The Lord of the Rings there is no hint that Glorfindel was reincarnated. However, this is somewhat foolish. If we strike the validity of things simply because they were not in one of the works published during Tolkien's lifetime, then the entire stories found in The Silmarillion cannot be true (and this is not simply saying they are not Tolkien's final ideas). Moreover, we have very firm affirmations of Glorfindel's reincarnation and coming to Gondolin in the two essays written at the end of Tolkien's life, and nowhere is this idea rejected. It is, nevertheless, perfectly conceivable that Tolkien would have rejected this idea. He has rejected other late ideas either because he did not like them in the end, or because they contradicted published text. Still, there is no indication of either of these, for the idea of the two Glorfindel's being the same is consistent with his mythos.
Combined with this complaint is the idea that Tolkien did not mention Glorfindel's reincarnation until over 15 years after The Lord of the Rings was written; therefore, it seems evident that he did not intend for the two Glorfindels to be the same while writing the saga. A comment by Tolkien only adds evidence to this claim: "Its use in The Lord of the Rings is one of the cases of the somewhat random use of the names found in the older legends, now referred to as 'The Silmarillion, which escaped reconsider- ation in the final published form of The Lord of the Rings" ('The Peoples of Middle-Earth "Last Writings"). It doesn't seem that one can really argue against Tolkien's claim. There is a preliminary text of "The Council of Elrond"in published in The History of Middle-Earth Volume 6: The Return of the Shadow that outlines the events of the Council which states, "Glorfindel tells of his ancestry in Gondolin.¹" This could at least show that the thought of the two being the same hadn't escaped Tolkien's mind, but Tolkien's own words still show that the name Glorfindel was used without thinking of connecting him (though perhaps his words above could be construed to allow some intended connection at some point during the writing; if it hadn't escaped reconsideration maybe the name would be changed, or maybe it the connection would be solidified).
Nonetheless, this still really doesn't matter in the final decision. The whole of Tolkien's writings are filled with instances where he wrote something and then later went back and connected it with his history. The Necromancer in The Hobbit, who, as Tolkien explicitly states in his letters, was not in any way originally conceived to be Sauron, is later in The Lord of the Rings identified as Sauron. If The Lord of the Rings had never been published, but had a fanbase like The Silmarillion does today, would we consider this connection to be valid? Certainly, even though it was made only after the publication of The Hobbit. Of course, just about everything that was written by Tolkien about Middle-earth after The Lord of the Rings contained, in some form, a connection to the published text that was not initially present in Tolkien's mind. Just as we cannot call these connections false—for in doing so we would be discarding the great literary efforts of of his ever-developing mythos—we cannot entirely challenge the idea that the two Glorfindels are the same. Tolkien says they are, very clearly, and ultimately, that really should be enough.
¹ Martinez, Michael. "The Wars of the Glorfindels." Suite101.com. 17 November 2001 http://www.suite101.com/article.cfm/tolkien/53015.