Under Construction.

Player killing in 4D is optional. Going PK is a voluntary choice you make, and to prevent newbies from rushing into situations that they cannot handle, there are three restrictions:

1. You have to REGISTER as a Pkiller first.
2. You must have remorted twice, and the cost is one bronze token.
3. You are only allowed to attack and kill players who also are registered as PK.

Players who aren't PK are protected by the code and cannot be attacked. When killed by mobs their corpses are also safe from looting.
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To become a Pkiller, just go to the Mayor's office in Olde Yorke and type REGISTER. After doing this and paying the fee, you get the label PK next to your class in the who-list and you're now allowed to kill, loot and steal from other players with the same flag everywhere in the 4d realms. (Except in the peaceful rooms like Recall.)
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Your score window will keep you up-to-date on how many PK-fights you've lost
and won. It also keeps track of your PK-points. Those points will rise if you
succeed in killing a higher level PK'er than yourself, or will go down if you
kill a person that is a too easy target for you.
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It is highly recommended not to register at a too low level and tier, since there are no level restrictions to PK, so this will make you the lawful target for any Pkiller in the Mud, most of whom will be much stronger and more ruthless than you. They will also be able to loot your corpse and maybe junk all, or some of your items.
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Going PK is a conscious choice you make. PK is NOT consensual - that would defy the purpose.
It involves running the risk of being PKed and junked, and the ability to do the same to other PKillers - EVEN if you/they are AFK.
The rest is PK Culture, which seems to vary over time, and is created by the PKillers themselves. Usually retaliation seems to breed more retaliation.
Note that you don't HAVE to junk everyone you kill, just because you can, and you don't HAVE to attack players who are AFK or far below you in strength - in fact many would call it a dastardly thing to do. Again it's a choice you have. And if you make that choice, it increases the risk of being treated in the same way yourself.
I also wish that people would remember that the purpose of PK isn't to settle OOC differences.
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PKillers are never fully safe anywhere. You need to always look over your shoulder, where ever you are. A Pkiller should never go AFK in an unsafe room and always sit down when squatting at Recall or any other peaceful room. If you cannot deal with these risks, you shouldn't be PK in the first place.
Don't go whining to the Admin about getting killed or looted too often. If you cannot handle the heat, get out of the kitchen - in other words, pay to get your PK flag removed.
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The PK flag can only be removed by a senior Imm, and you will have to pay the fee of one Silver Token to have it removed.
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Players who are persistently obnoxious, and for instance keep taunting PKillers verbally under the protection of being non-PK themselves, run the risk of having a PK flag set on them by imm intervention.
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For players, who like to try the taste of PK without taking the step of registering for it, there is also the Arena and the Gladiator Arena, where you can fight other players without risking the loss of exp or equipment. In the common Arena you fight with your normal char and equipment, in the Gladiator Arena you'll get access to one of the 16 Gladiator Chars, who are all set at the same race, level and tier, but are different Classes.

Any imm can start a common Arena, and will usually do so on request. There are also some areas in the game that are flagged Arena, for instance parts of the Tournament Place.
For the Gladiator Arena there are specific rules and conditions, described in detail under that link.
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If someone else feels like writing something more detailed for this, for instance elaborate over the planned code changes, please go ahead. Players can send it to Diandra by mudmail or note.