He didn't run out of stamina, he was just trying to stun me out of it and button mashing. He pulled it off successfully a couple seconds before, but it was better timed the second time and the hyperarmor kicked in by the time his sword connected, and he had already committed to the second swing.
That's the Bloodletter, you get it in the dlc when you get to the fishing hamlet and get the cell key from Simon in the fishing hut (or the unnamed corpse there if you kill him). Afterwards you go back to where you fought Ludwig and kill Brador in his cell. I personally call it the giant Q-tip, the great ones have dirty-ass ears
