This plugin allows you to enter a simple command (below) ans start playing this assassin gamemode yourself! The assassin has the task of killing the player, and can do so in a single hit, but there's a catch. Any time the player has their cursor on the assassin, the assassin is frozen, and can't move or hit the player. They can still break blocks and such though.
/assassin add <name>
/assassin remove <name>
