Disclaimer: Star Wars belongs to Lucasfilm and Disney. I make no money out of this and am merely playing in the sandbox that George Lucas created.
Their duel only got more intense. Vader was fully on the offensive and his opponent totally on the defensive.
Well, what more could you expect from a Soresu Master?
With every attack, every blow, his opponent would block it. As quickly as he attacked from the side, his opponent skilfully blocked it with a certain mixture of anger and hopelessness that Vader had never before seen in his eyes.
This only induced more rage in him, for a reason he could not accurately place. Drawing on the power of the Dark Side, feeding off of his new found fury, he ruthlessly crashed his light-sabre on his opponent.
The Jedi blocked that too.
As ash from that damned volcano rained upon them, it became clear to Vader that the Jedi was no walk in the park.
(He knew he should have expected as much.)
Then, quite suddenly, the volcano below the bridge that they duelled upon erupted.
And then -
Victory.
To Vader this happened in slow motion.
The Jedi was thrown backwards, almost certain to fall into the boiling, writhing lava.
Wait - almost certain.
Obi-Wan survived.
Anakin had been conditioned to save Obi-Wan's life for thirteen years. Vader did not forget.
And the Sith's eyes widened in shock.
He had caught Obi-Wan from falling.
He could see Obi-Wan's eyes were in shock too.
He pulled him up, and in a moment, forgetting what had happened, he said:
"Looks like I saved your ungrateful arse for the fiftieth time, Master."
Notes: constructive criticism welcome.
[Edit: I didn't post disclaimer. Done now.]