These are my thoughts. First of all, I have to clarify that I have not played WoW (though I did play Warcraft 2 and 3 extensively, the games where that class was inspired from).
The overall design seems about right, if a bit messy at times. My main suggestion is to turn it into a paladin Archetype. You already have pretty much the same class skills and the BAB and good saves are the same. It is not clear if these guys have proficiency with shields.
The mechanic for regaining spent Rune points is a bit hard to pull off. My suggestion would be to tie it to critical hits/kill hits with negative energy, then give the class a way to use negative energy with its attacks.
I would also pump the damage dice of Death Coil to d8s, set it's damage type to negative energy and clarify that it heals any undead it targets (basically working like "Cure/Inflict" spells).
It is my understanding that death knights are undead on the warcraft universe, so you might add a clause on that too.
My 2 cp.
