ARGUMENTS: [player/NPC name]

The "kill" command initiates combat with a player (or an NPC).

If used with no argument, "kill" will attack your primary target.  If your
primary target isn't in the room, kill will instead pick a random player (not
an NPC) in the room for you to attack, among all enemies in the room.  If there
are no players in the room, but there are NPCs, then a random NPC will be
chosen to attack.  Invisible players will never be chosen unless they are
attacking someone themselves.

With a player (or NPC) name, "kill" will initiate combat against that player.  
Invisible players can be attacked in this manner.