This is what happens when I contemplate a song and its relevance to Star Wars, and am encouraged by multiple people. Thanks to dogmatix, hamelin-born, and the anonymous whose asked questions on tumblr for making this plot bunny the actual beginnings of an AU.
The Sith is waiting for them when the hanger doors open, clad in black and wearing an aura of menace like a dark cloak, one deep enough that Obi-Wan doesn't doubt it can be felt by anyone near by. It's curious neither he nor Qui-Gon had sensed it before the doors opened, though, as if the Sith had been waiting for them to find him before he let them feel just how much danger awaited them.
The Sith is patient enough to let Amidala and her entourage clear out of the way, focused entirely on Obi-Wan and Qui-Gon. He even gives them a moment to shed their cloaks, so they won't get in the way of the fight. His is still the first saber lit, a double-bladed staff that makes Obi-Wan hide a wince. This is not a weapon they've trained to encounter, though he has hopes they'll prevail regardless.
When he strikes the first blow, vaulting over the Sith's head and nearly losing his saber-hand for it, Obi-Wan wonders how deeply they've underestimated the Sith. The ferocious attack is hard to keep up with, and they can't keep the Sith between them despite trying.
Obi-Wan thinks he hears a crack when the Sith kicks Qui-Gon's wrist, and Qui-Gon goes down with a hiss of pain, if only for a moment. It's long enough for the Sith to push Obi-Wan several feet across the hanger, toward another pair of massive doors that lead, if he remembers correctly, to Theed's power station. Not an ideal place for a fight like this, and Obi-Wan tries to circle around to push the Sith back into the hanger.
The heat of the Sith's lightsaber singes his hair as Obi-Wan has to tuck to avoid having his head cut in half, and he falls back a step, leaving the way open for the Sith to take several steps closer to those doors. Not a retreat, but a lure onto ground he's chosen, and Obi-Wan has a very bad feeling about what will happen if they are drawn beyond those doors.
Panting, he lunges, drawing on the Force and his own worries to push himself harder as he re-engages the Sith. Qui-Gon is there beside him again a moment later, showing no signs of injury, though Obi-Wan suspects he's using the Force to hold off pain and to keep any cracked bones in place.
A piece of debris goes flying through the air, only barely missing Obi-Wan before it crashes into a control pad, the doors opening easily behind the Sith, giving him a chance to move them into the huge open space of the power station, criss-crossed with catwalks that Obi-Wan can only think of as death-traps.
They're barely inside the door when Obi-Wan is kicked, and the taste of blood floods his mouth as his teeth catch his tongue. He stumbles and falls, rolling away to get back to his feet, and spitting out a mouthful of blood as he does so. He doesn't stop, can't stop, just darts back to where Qui-Gon is fighting with every bit of speed and strength he can wring out of his body and the Force.
It won't be enough, not here, and Obi-Wan catches the Sith's staff before it can carve into Qui-Gon's shoulder, pushing him back with a Force-shove. Only a fraction of a second of breathing room, though, before the Sith is back on them, blocking their blows as they try to end this here, now.
Qui-Gon prevents another would-be decapitation, and Obi-Wan doesn't have time to even give him a smile of thanks, pushing forward and toward one of the smaller console-platforms rather than the catwalk itself. The Sith smiles, retreating to the lit edge, spinning his staff once as he waits for them to attack him this time.
Nearly impaling himself on the Sith's staff isn't Obi-Wan's intent when he lunges, and he doesn't like that he has to hastily throw himself backward to avoid that, leaving the Sith room to backflip and land on the catwalk itself. Time for the Sith to be ready when Qui-Gon and Obi-Wan follow, and for him to catch both of their lightsabers on one blade of the staff.
The fight gets no better from there, chasing the Sith down the catwalk only for him to leap up to another, forcing them to follow him higher and higher into the station. Until there's nowhere to go up, and they're fighting close to one of the great columns of plasma that terminates above their heads in the collectors.
Obi-Wan feels a tiny whisper in the Force only a split-second before the Sith twists and spins, delivering a vicious kick to Obi-Wan's ribs that pushes him off the catwalk, and cracks at least one in the process. He fails to catch himself on the first catwalk he hits, and rolls off it to drop onto another one, barely grabbing onto the edge to keep himself from falling another level. The jolt sends pain screaming through Obi-Wan, and he barely clamps down on that enough to look up.
Qui-Gon is retreating, step by step, and barely keeping up the green blur of defense. It falters a moment, and Obi-Wan reaches out with the Force to yank ferociously at the back of the Sith's robes. It gives Qui-Gon enough time to barrel into the Sith, shoving him further off-balance before Qui-Gon backhands him, sending him careening off the catwalk.
Obi-Wan can hear the thud from where he's dangling, and he struggles to contain his pain long enough to pull himself up. It only leaves him in more agony as the motion pulls at his ribs, and he pants, listening to the sounds above him. The Force is screaming at him that something is wrong, that this is going to go badly and soon, and he wants desperately to stop that.
Trying again, he draws on the Force to leap back onto the catwalk he's hanging from, and looks up in time to see Qui-Gon land near the Sith - and the Sith's saberstaff to come up and carve into Qui-Gon's side before he yanks it upward savagely.
Not enough to kill immediately, but Obi-Wan can feel the searing pain along the bond with Qui-Gon, and he hears someone screaming in negation. Anger sings in his veins as he uses the Force to direct his leap again, landing just out of reach of the Sith's saberstaff. Power follows the anger, strength enough to ignore his ribs and charge at the Sith with his saber - he can't remember picking it up - lit and at the ready.
The Sith matches him blow for blow, though he keeps retreating. Toward another door that Obi-Wan can't recall now if it leads to another level of the hanger or somewhere else in the palace complex, and doesn't entirely care. All he wants is to see the Sith fall, to die before he can kill someone else, and best that he dies at Obi-Wan's hands.
Qui-Gon is still in the back of his mind, and Obi-Wan can feel his Master's concern and worry weakly beneath the pain that is most of what he feels from him. He shouldn't be worrying about Obi-Wan, only about living long enough to get a medic to him.
He lets himself sink further into the Force, using the anger - rage - at the harm the Sith has already done feed him more of that same power. Drawing it into himself and pushing it into the battle. Moving fast enough that he's distantly aware the sabers will look like nothing more than blurs of blue and red to observers, though he can see it all as if time has slowed.
Block and parry and duck and leap. Strike and lunge, and there. An opening, almost too brief and too small to catch, but enough. Just enough to slash through the hilt of the saberstaff, and past it to cut through black robes and the flesh beneath. Carving bone and organs with equal ease, severing the spine at the same time he slices through the Sith's heart.
His world explodes with pain and sound, and Obi-Wan drops to his knees, his lightsaber skittering away from him when it falls from nerveless fingers. Obi-Wan doesn't know what the Sith did in his dying seconds, but it feels like he's suddenly surrounded by a crowd that is jeering and shouting. Loud enough that he can see phantom shapes out of the corners of his eyes that vanish when he turns to look at them.
Obi-Wan only realizes he's folded over with his face nearly against the floor when he feels a warm hand on his shoulder, familiar and welcome, and he gasps out relief that quickly becomes terror when he looks up to see nothing. He can still feel Qui-Gon's hand on his shoulder, can hear him trying to speak comfort that Obi-Wan can't make out past the rush of blood in his ears, but there's no one there.
Shaking his head, Obi-Wan knows he needs to bring himself under control, to let the fear and pain and anger out into the Force, but he can't think with the cacophony in his mind. Can't focus past the churning in the Force itself, the darkness that swamps his senses, except to know he needs to move. Move, run, hide.