In The Lord of the Rings, several dark and mysterious corners of the world take the story outside of the relatively linear conflict happening between the free peoples of Middle-earth and the forces of Sauron. The Dead Men of Dunharrow and the journey Aragorn takes to hold them to their oaths is the most obvious example, but it wasn't the first time members of the Fellowship had encountered ghosts. While journeying to Mordor, Frodo, Sam and Gollum have a desperately difficult journey through an area known as The Dead Marshes, with the former almost losing his life as he attempts to travel through.
Throughout the history of Middle-earth, there have been countless battles between the forces of good and evil marching under different banners. Within those, some stand out as more momentous and significant in their results. As detailed in The Silmarillion, the Battle of Unnumbered Tears and the Battle of Sudden Flame are marquee moments that provide some of the cruelest and most crushing losses for the Elves at the hand of the evil Valar Morgoth. Following his defeat, Men and Elves fought to banish his servant Sauron during the War of the Last Alliance and it is from here that the Dead Marshes accumulated its dead.
The Bodies in the Dead Marshes Come From a Great Battle
The Fellowship of the Ring depicts the most critical part of the closing moments of the war, the Battle of Dagorlad. Elendil is struck down by Sauron before his son Isildur takes up his father's broken blade and strikes down the Dark Lord. The scene is used to illustrate the fate of the Ring after Isildur fails to destroy it, but this wasn't the only lasting legacy created during the Last Alliance's victory. Many Men, and more importantly, Elves, died during the battle despite their eventual triumph. The land was permanently altered, with the Dead Marshes serving as the most starkly disturbing reminder of that time. The fallen lie in wait, just beneath the seemingly shallow waters, waiting for travelers who make the mistake of following the inextinguishable lights.
The marshland hasn't simply retained and preserved the bodies of the dead; it also keeps their souls. This is unique in the world of Tolkien as the sites of even the bloodiest battles in Middle-earth, though they can remain tainted by their pasts, eventually fall into a state of relative normality. So what is the reason for their preservation? It was never explicitly explained by Tolkien in his writings, but there are some precedents elsewhere that could serve as reasonable ideas for why the Dead Marshes exist.
The Ghosts May Be Elves Who Didn't Make It to Valinor
It could be that those who fell during the Battle of Dagorlad became what Tolkien referred to as Houseless. Essentially, Elves who were slain during battle weren't obliged to go to their version of the afterlife, a place known as the Halls of Mandos in Valinor. If they refused this call, their souls could remain in Middle-earth but separate from their bodies. The Elves, feeling as though they wanted to stay and continue to fight, unaware of the true consequences of their deaths, could have become trapped in the marshland indefinitely.
The bodies also clearly reference Tolkien's experiences in the First World War. The author was present at some of the war's bloodiest and most psychologically devastating battles, including the Somme. The corpses of fallen soldiers from battles long past, lying in the mud and water, is one of the most potent and resonant uses of imagery in the entire story.