No one is playing dark knigth in pvp becouse they suck rigth now, adding a new pvp Skill that share dmg with targeted player. the animation could be a blood beam going from the dark knigth to the targeted player. the effect of it could be.

Blood Bond : redirect 30 % of incoming damage from the targeted player to the caster for 30 sec. cooldown. 2 min

this could be a nice thing both in pve and pvp. i hope they add this soon so dark knights become an actual choice.
but more likely is they will add something like this in next expansion but you could always hope