After the events of the Mithras Directive Deadpool goes quite insane. Specifically he starts seeing white rabbits everywhere and a woman pouring some type of liquor into some other drink that it wasn't supposed to mix with (i.e. Bourbon into milk). He sought help from Dr. Bong to interpret and/or end this psychosis. He came to the realization that he knew this woman in his hallucinations.
After Being sent on an assignment and battleing Bullseye he grabs a real life, spitting image, of the girl from his dreams and reveals that she is Mercedes Wilson, his wife. With the next couple issues this is explained. Enter T-Ray, Deadpool's arch-nemesis. Deadpool is responsible for her death.
The story was that T-Ray (in the past Wade Wilson) and Mercedes lived a very peacful and secluded life in the far unkown of Maine. During a really snowy and cold period they were unable to leave their house for quite a while but when outside at some point they found a man in a river and took him in a cared for him for weeks. This man turned out to be a ruthless murderer and attacked them when he was rested and able to leave their home. This mercenary was Deadpool in his early years. He succesfully killed Mercedes and assumed he had taken out Wade (obviously he hadn't).
After this shocker was revealed T-Ray, taking Deadpool and Mercedes prisoner. T-Ray explained he was the peaceful husband of Mercedes living in Maine (and that HIS name was actually Wade Wilson), and that Deadpool was the ruthless killer they pulled out of the river. Deadpool laughed this off (literally) and pointed out to T-Ray that while he was setting up this revenge and bringing Mercedes back from the dead just to torture Deadpool, Deadpool had been trying to be a better person and helping people. She left with T-Ray but gave Deadpool a second chance at life.