We're gonna pull from some Expanded Universe stuff here, now classified as Legends, I guess? Star Wars canon is crazy in general, let alone with all the factual rearranging since it was sold. So, until the new movies prove otherwise, this is how I see this going down. How Luke's lightsaber gets from C'Baoth, who probably won't show up on film ever, to Maz Kanata is a different story.
1/ Mount Tantiss, the private storehouse of Palpatine. Once a hidden military base on the planet Wayland, it has been repurposed as a repository of artifacts that would most assuredly be better off not under the ownership of the Empire. One such container is marked B-2332-54, holding the severed hand and lightsaber of Luke Skywalker, collected after the duel with his father, Darth Vader, on Bespin. Standing in the cave entrance, with the faintest hint of large, complex machinery behind him, is The Guardian of Mount Tantiss, selected personally by Emperor Palpatine. He wears the dark, hooded robes of a Sith and brandishes a red-bladed lightsaber.
The Guardian (Cap): Of course I have felt it. Watching over this place, how could I not?
The Guardian: I knew, someday, someone would come.
2/ Joruus C'Baoth, the insane clone of an already questionable Jedi. Even though he is clearly the aggressor, and we've been trained through years of Star Wars media that RED means BAD, the lightsaber he comes swinging in with is green. He certainly looks insane, with his beard and robes and hair all flying around him.
C'Baoth: Today's the day!
3/ As the two Jedi clash, what we see instead are the results of their fury on the surrounding terrain and wildlife. Above the treetops, into the air, escapes a flock of stana. They are large, black, vulture-like creatures, and even they know better than to feast on the corpse of a corrupted Jedi.
SFX: FSHZZ (multiple)
SFX: kree kree (multiple)
4/ C'Baoth pulls his lightsaber out of the gullet of The Guardian. The Guardian is falling backwards, his hands opening reflexively, causing his saber to extinguish as it drops. His hood begins to fall back from his head, revealing just a trace of his white beard, but nothing else.
The Guardian (Cap): I cannot prevent what happens next.
5/ In the background, Joruus C'Baoth enters the cave. He will get what he came for, and the story of The Guardian of Mount Tantiss will ultimately be lost. Later, many will confuse him for The Guardian himself, claiming that this battle was purely inside Joruus' deteriorating mind. In the foreground, his hood fallen completely, as has the rest of him, is a twisted clone of Obi-Wan Kenobi. Whatever cruel joke Palpatine constructed, and to what end he constructed it, is inconsequential. It's over now.
The Guardian (Cap): I can only pray it ends up in the right hands.