Vampiric runeblades are empathic weapons given to new death knights by the Lich King Ner'zhul or one of his designates to turn them into death knights. Once the runeblade is used to turn a hero into a death knight, it becomes that knight's personal weapon and is slowly charged with the life energies of those injured and slain thereafter.
Death knights are now few and far between, but their legacy lives on. The runeblade is one example of their terrible power. It channels life force, which bestows a kind of life upon the blade itself. Though not very intelligent, the runeblade has a strong survival instinct.
When a death knight who owns a runeblade is destroyed, the sword dampens its powers, taking on the appearance of a nondescript magical weapon. It uses its empathic powers to manipulate its next owner — encouraging feelings of possession and anger, discouraging kindness and altruism. Once it gains enough control over its owner, it encourages him to seek out the secret places where death knights still dwell. Only then can the runeblade accomplish its goal of gaining a new master.