A monster player kill (also MPK or mob-player kill) is an act of dragging a large train of monsters onto a victim player who is either doing nothing or is already fighting. The idea is that the monsters will then attack the victim, who will be overwhelmed by sheer numbers and will die as a result. The term is often used as a verb (ex: "I just got MPKed in the Plaguelands.").
MPKs are considered griefing or harassment, as they lead to at the very least a serious problem for the victim suddenly being thrust into combat when they were unprepared. More often than not, they result in the death of the character subjected to them. They are also notoriously difficult to successfully prove.
Normally, when a player flees from mobs, the mobs will return to their spawn positions after a short pursuit. However, during the course of their pursuit, if the mobs see another player who is fighting their friend, instead of returning to their spawn position, they will go assist their friend.
Due to the difficulty in predicting when mobs would give up their pursuit, MPK is normally difficult to execute for most players. However, classes such as hunters and rogues can drop aggro at will with Feign Death and Vanish. Therefore, it is relatively easy for those classes to commit MPK.