Assassin is a casual multiplayer format for Magic: the Gathering where the players are randomly assigned "targets" to defeat.
Players are free to attack whomever they want. However, victory is not determined solely by who the last player is, but also by who manages to take out the most marks over the course of the game. Players score points for delivering the finishing blow to their assigned target as well as being the last survivor. Defeating another player grants you their mark, and thus a new target to attack.