Player killing (pk) is player vs player (pvp) combat, as opposed to player vs environment (pve). There are two types of pk fighting: arena pk and temporary pk.
main article: Player killing arena
From a waypoint type pkok setup <arena number> or pkok join <arena number> if an arena has been setup.
This allows pk fighting outside arenas. Type pkok <char name> to register and who pk to see which characters have registered.
Alter aeon was conceived as pve mud and support support for player killing was added later.