Hmm I have an interesting idea. What if this skill was given to paladins at 30 or 50 and they would be given the choice to sacrifice one of their friends or family (which would be a real player ig). Only then will they be able to form the most unholy of pacts with the Gods...
This could lead to another qclass.
Imo dks these days have gotten way too soft. Sitting and talking goodies into believing they're actually not evil? I mean come on now. What kind of villain are you? Dk's are THE EMBODIMENT OF EVIL! Maybe we do need to introduce death knights this way.
I like your idea, and I like the original sacrifice idea. For Death it could be your soul, greed say like 1mil gold, combat a unique weapon/armor, etc etc. Would be great IMO.
One thing I would suggest is to make it a type of quest deal, where you know you have the ability to sacrifice something, but the where and what are something you need to figure out, like a greed cleric saccing 60k gold gets a buff, but he has no idea that if he did a full mil that it could be so amazing.
On second thought, making DK a quest class is cool and all, but doesn't evil have enough options for Q-class?