Have any bow equipped and aim it. Then, get close to any fire source (fireplace, brazier, torch) to light the arrow on fire and add fire effects to your next shot. Since a fire source is not always available, you can always use your torch. Throw your torch on the floor, then light the arrow on fire. If you shoot a fire arrow into a red jar or container, it releases an oil that sets the nearby area on fire. Lots of guarded areas have this, as well as all small boats. It is a great way to quickly kill a lot of enemies.