This command will toggle you between two modes: fatal, and merciful.
@fatal -> Combat switched to fatal. @fatal -> Combat switched to merciful.
The main difference between these two modes is your intent. In merciful (often described as @merciful), you will not actively try to kill your opponent and are instead seeking to knock them unconscious, whereas when you are in fatal mode you may land a hit that kills your opponent outright.
Note: Combat is not a safe activity. Even if you are in merciful mode there is still the potential to maim or kill your opponent. Critical hits to the head or chest may result in organ damage or death. For safer combat, see @spar.