From Lord of the Rings, Celeborn is the King of Elves.
Celeborn was a Sindarin prince of Doriath, being the grandson of Elmo, the brother of Thingol; thus, he was the grand-nephew of the King of Doriath.
Celeborn is even one of the last Elves we hear about in the Fourth Age that follows "The Lord of the Rings." The Prologue to "The Fellowship of the Ring" even ends with the line, "It is said that Celeborn went to dwell [in Rivendell] after the departure of Galadriel; but there is no record of the day when at last he sought the Grey Havens, and with him went the last living memory of the Elder Days in Middle-earth."
Despite the frequency of mentions, Celeborn remains hazy for two reasons. First, there isn't much detail about him or what he does. Second, when there is detail, it tends to be inconsistent over the course of Tolkien's endless drafts and rewrites. Even so, there's enough there to paint a basic picture.
Celeborn and Galadriel meet in the First Age, fall in love, and eventually head east, looking for greener pastures. Early in the Second Age, they have a daughter named Celebrían. The major difference from the show here is that Celeborn doesn't get lost in a war. He's with Galadriel the whole time, and the two quickly become one of the premier power couples in Middle-earth. They live in Gil-galad's kingdom for a while at the beginning of the Second Age and then relocate further east again, eventually ending up in Eregion with future ring-maker Celebrimbor.
Celeborn and Galadriel split up (temporarily)
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There is a period in the Second Age (when "The Rings of Power" takes place) where Celeborn and Galadriel do split up. At one point, Galadriel and Celebrían pass through Khazad-dûm to visit the Elves of her future realm of Lothorórien on the other side of the Misty Mountains. Celeborn has a major beef with the Dwarves and opts to stay behind.
Before the family can reunite, Sauron brings war and devastation to the region. Celeborn leads an attack on Sauron's forces before hooking up with an army commanded by Elrond, and the pair of outmatched Elvish generals retreat to the area where Elrond founds Rivendell. Once they've got their Elvish fortress and refuge up and running, Celeborn hunkers down and hangs out until Galadriel seeks him out there after the dust settles from the first tussle with Sauron.
After this, the love birds stick together again. Eventually, they head south and live in the area of future Gondor for a while. They may have wandered a bit, too, although J. R. R. Tolkien is unclear with the details at this point in the story. The one thing we know is that by the Third Age (after "The Rings of Power"), they end up in Lothlórien, where they become the Lord and Lady of the Golden Wood.
And that's it in a nutshell. Keep in mind there are countless details and contradictions that are left out, but the general gist is that Celeborn and Galadriel fall in love in the First Age. They spend the Second Age starting their family, slowly migrating east, and separately resisting the forces of Sauron the first time he tries to conquer the continent. They eventually reunite in Rivendell, and they resume their wandering right into the Third Age before finally settling down in Lothlórien.
How could a returned Celeborn factor into the Rings of Power narrative?
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Okay, to recap: Celeborn from the books is an interesting, underdeveloped character — but he's there with or near Galadriel throughout the action. In "The Rings of Power," though, they've written him right out of the early narrative. In Season 1, the guy is technically MIA and may as well be dead at this point.
If the new rumor is true, and Calam Lynch of "Bridgerton" plays the role of Galadriel's trophy husband, how could they work him back into the action? The major hurdle is coming up with a reason for the character to have disappeared for centuries without reconnecting with Galadriel. Is he in prison somewhere? Does he have amnesia?
The fact that the couple spends significant chunks of time apart in J. R. R. Tolkien's texts makes this easier to address. Middle-earth doesn't have high-speed internet, and it's not like Galadriel is stalking Celeborn's social profiles. The Elf-woman has clearly spent most of her time on the move chasing after Sauron, too. Perhaps the two have been wandering through various trials and tribulations and simply haven't reconnected yet.
If the new rumor is true, the question of how they will introduce Celeborn and explain where he's been will be top of mind for viewers as they tune in for Season 2 of JD Payne and Patrick McKay's Middle-earth adaption. There's also the curiosity of the couple's daughter (a character that has already been teased by Galadriel actress Morfydd Clark). Here's hoping we get to see it all unfold on our streaming screens sooner rather than later.