During the events of The Crucible:
Forged in the underground forge of Brock in Asgard the blade known as Nightmare not only replaced Lady Death's previous sword, Darkness, but was made from the shattered hilt of Darkness melted down and bound with ever spell that Brock knew as well as many he'd never tried before.   
                                                                                                                                            
It was poured into an unusual shaped mold that was commissioned by Odin many years previously, with instructions not to be used until the time was right. With Asgard on the brink of destruction Brock decided that this was the time.  Brock knows enough to know it melds to its master and to take its handle is to joined with it. He handles it with gauntlets by the blade only to get it to Lady Death. The Chaos being Antigone teleports Brock and the blade to Wrathworld to give it to Lady Death but he drops it when his thumb is severed. Vex appears to have picked up the blade with a thin gloved hands but, before it can bond with her blood, she drops it before being banished to earth with no memory of who she is. 

Genocide strides away and as the dust clears Brock brings nightmare to Lady death's apparently lifeless body. He asks who will wield the blade and her hand reaches down and declares that she will. With the sun destroyed its only a matter of time before this place is in ruins. She is re-energiezed and heads off after her nemesis. As she goes she doesn't even notice that the arms of the imp that decorates the hilt and forms the guard clamp inwards and the little face sucks the blood from her hand making the bond between them complete.  The sword utters its first words: "HA HA HA! DEATH BECOMES HER!".  She uses the sword to help in her battle to destroy Genocide. Back in Asgard she spends increasing time alone with the sword and abruptly decides to return to Hell.
 
During the events of Wicked Ways Nightmare continually requests to draw blood with its blade, even though its perfectly capable of death without being drawn. At first she is able to resist its calls but faced with a demon she has already overpowered she gives in and takes it's head with the blade. Later in Gehenna she destroys a soul in torment with the blade much to its delight but its greed means it asks for more. Cremator views the exchange and expresses his concerns about a blade that suggests and controls. 
 
Lady Death is even unable to spend one night without the sword at her side and suffers withdrawal symptoms and can hear its voice in her heave even though its locked in a trunk. Later it saves her life by twice reanimating her unconscious form to carry on battling.
 
When Lady Death discards the blade Purgatori later claims the blade for her own and suffers many of the same issues that Lady Death had with the blade insatiable blood lust, even that of its owner, and tendancy to not care who's blood it spills. 

Powers

The blade is a vampire blade and always seeks to take lives and should someone master and discard the blade, they become immune to it. It can kill without being unsheathed using the ghostly spirit tendrils that emanate from its blade, often have dragon faces at their tip, but prefers to use its edge to draw blood.  
 
The sword is sentient and communes directly with its holders mind and encourages them to more violent acts than the situation calls for.