While looking for this fellow I learned something noone posted yet: It seems there is some area effect around the cave the Vampiric Deathknight or his ph are located in. This effect will warp not only the mob but also you merc. This effect is effect even if no mobs are in the cave. Just enter the cave and your merc is warped across the zone. The end point of the warp is not completely random but one of a few possible end points. If you try it with a merc a few times you will soon know them all.
To sum it up: Must fight inside the cave at the spawn point and without the help of your merc.

Fangbreaker Hunting Fetish
Gilded Targe of Kulthar
Rusted Necklace
Spell: Wildmagic Burst
Vampire Blood