Swordplay. I did a brief series on this a while back, but if you didn’t catch that, here’s a brief summary. Swordplay of various types is so common in fantasy and historical fiction that you absolutely have to be able to write it well, which means you need to take into account various basic considerations:
What kind of sword is the character using? This is mostly something to reflect a character’s type, origins and role in the story, but it should also be appropriate for the way they’re going to use it. Anyone using one of the two handed Scots claymores like a small German messer in a story is obviously a lot stronger than me.
One hand or two? One sword or two? Does your character hold the sword with a single hand or put both on for more power (with many swords, from the Japanese katana to the hand and a half sword, there is a choice)? Do they use a second matched or shorter blade in their off hand? Do they use a shield? Remember that a single sword used alone in one hand is actually at a disadvantage against many other options, but offers more mobility than the two handed option if nothing else is available. Your fantasy sword master will know that.
How do they fight? Do they fight with neat parries and ripostes? Devastating counter cuts with the angle just right? Fluid motion to avoid the blade? A solid, unyielding defence using a shield? Do they use lots of sneaky feints or batter their way through defences? Do they kill quickly or cut at the extremities to wear an opponent down?
It might sound like all this is about demanding an intricate knowledge of the blade, but it isn’t. It’s about knowing your characters, and getting that character across even in action oriented scenes. Conan would never fight like the Grey Mouser, for example.