Dueling State / Form
I propose a new addition to the vampire shapeshift abilities. For simplicity I will refer to this state / form as Duel Form.
INITIATING A DUEL
Duel Form will require 2 players to cast. A duel will be initiated by a challenger performing an animation that is akin to "Expose Vein". A second player must accept by interacting with the challenger. At which point both players enter the Duel Form with full health regardless of the amount of health they currently have in normal status.
Duel Form can only cause damage to other players also in Duel Form. Duel Form will prevent all damage from other Duel Form players that would otherwise decrease their health below 1. Players in Duel Form are not immune to damage from non Duel Form players and can be killed by non Duel Form players.
At the conclusion of a duel a player would perform an animation that suggests defeat and return to normal status with the exact same health they had when entering the Duel Form.