You burn them, then you die
If you try to duel someone with a sword that is heated to 800°C, and you manage to get the first hit, what will happen is that...
- you will cause a nasty burn wound on someone
- you will die, because your blade will bend... and while you are reeling from
surprise, the enemy will sucker-punch you and kill you
Steel at 650°C has lost 2/3 of its strength. At 800°C it is down by 80-90%, or more. That is so low that any impact, stab or other exertion of force on it, will cause it to deform. And of course it will not keep any kind of edge.
You would have known this a long time ago, because on the first impact against cold steel or a training dummy, you would have seen this instantly. A sword heated to those temperatures will deform as soon as you try to use it, leaving it bent or badly dented. And of course the blacksmith will let you know this because they are intimately familiar with how iron and steel soften with temperature.
As a side-note: this is what brought down the World Trade Center towers in 2001. The fires did not melt the steel cores of the towers, but the cores did get heated enough to lose their strength, leaving them unable to carry the weight of the towers above the impact zones. This meant that the outer skin was left to carry the entire weight. With one side of the outer skin smashed by the planes, and the floor beams starting to sag (for the same reason: losing strength from heating) and pull on the skin from the inside, the outer skin eventually gave out and the collapse started.