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.
I have had this exact thing happen while trying with a merc.
The first time the mob warped to the same location as my merc. The second time I stood on the spawn and killed it without my merc who has warped to another location in the zone.
He spawns in the vampire caves, right near the Hills of Shade ZL. He is in the southern-most cave, and his placeholder is usually "a vampiric reaver" or "a vampiric healer". He is the only enemy in the room. However, if you attempt to pull him out of his cave, he'll warp to a random point in the zone (seen him end up near the Mistwynd festival, near Bloodmoon Keep, etc.) You'll have to fight him where he stands.
No idea on his PH's, but I was clearing the Fearfang camp when he walked right through it. I had not killed any undead, so he must have a large wandering area.
Useful information? You mean like someones whining about a lack of useful information? If you did not find other peoples information to your liking, why not go out and discover things for yourself and post what you find.
Currently on Quellious you have to fight both PH for this mob at the same time.
You MUST engage without casting, if you cast on either mob both will turn around, path into the wall and then warp, never to be seen again! This has been going on for some time, have bugged it but no fix, and not holding breath.
Once engaged you can cast on the mob you are fighting, as long as you are standing toe to toe with it. If someone else gets aggro mob will turn around, run into wall and warp again.
This is a total PITA but IF you can get the mobs to stay and spawn, the deathkinght is a very easy kill.
About %50 of the time, if you engage one the other will just warp away so that you only have one to fight. I have not yet figured out if it is the one on the east side or western side that is PH, as we have only got deathknight to spawn once in several attempts.
He warped to a place in front of Bloodmoon Keep, north side of the river. He popped in our group, when we fought some Shadowmanes. Don't know, if there are some other points, where he can warp to...