In life, Mortem has always been an unlucky guy, even more than your average demigod. I mean, his name literally means Death Destruction. In fact, his bad luck began about a year before he was even born.
His mother never had one of those standard jobs. She was one of the people who set up those suicide machines. She worked for a company called "Thanotics" which supplied euthanasia for victims of deadly diseases, where she would bring and assemble the machines and show them how to use it. After the suicider killed him/herself she would remove him and re-collapse the machine. She often had doubts about her job, and really the only reason she stayed was because of her boss.
Thanatikos was none other than Thanatos, the Greek god of death. He had pity on those who commit suicides, so he made Thanotics as a calmer, more peaceful way to go. He knew that if these kinds of services weren't around then there would be far more gory deaths, which he doesn't like. The serum he developed that got injected helped ease their way through to Erebus. It also made their souls satisfied, so whatever had made them commit suicide didn't hurt them as much.
Anyway, Malady had fallen head-over-heels in love with Thanatos and spent almost all her time - when she wasn't working - fantasising about him. Noticing the lust she had for him, he decided to take advantage of that, and he made the first move. The pair started dating, and they eventually had sex, resulting in Mortem. Once Malady was 2 months pregnant, Thanatos told her his true identity. Shocked, Malady committed suicide later that day, using the very machines Thanatos had invented. Sensing her death, Thanatos rushed to the machine but it was already too late. Thinking quickly, Thanatos created a quick caesarean section and removed Mortem from her womb. Trying to think of what to do, he remembered what his cousin Zeus had done with his kid, Dionysus. As fast as he could, he slashed open his thigh and sewed Mortem in.
4 months later, Thanatos removed Mortem and left him at an orphanage. He was given in a basket on top of a folded set of a leather jacket, black singlet, sunglasses, black jeans and black sneakers. Next to him was a locked book, with a name on the front: Mortem. The orphanage decided that was his name, and took him in. By the time he was 11 he first developed his powers. One of the older kids called Max befriended him, and the two of them became good friends. When Max was on a school trip, Mortem encountered his first monster. It was a hellhound pup, that had been drawn towards his power. At first, it tried to attack him, but the moment it readied itself to attack Mortem looked at it with his pitch black eyes, and the hellhound stood down.
The next day, the hellhound approached Mortem again, but instead of trying to attack him it walked right up next to him and hung around him. Even though the hellhound was at least a head taller than him everyone only saw a small mastiff pup. The next day when Max came back he was shocked to see the hellhound lying down next to Mortem, while Mortem patted his head. Max tried to take Mortem away from it, but the hellhound bared its teeth and snarled, making Max jump back. Mortem scolded the hellhound and told him it was fine.
The first time Mortem was properly attacked was on the day of his 12th birthday. A flock of 5 Stymphalian Birds attacked him and the other 12 and 13-year-olds who had gone to the park with him to celebrate his birthday. The Stymphalian Birds attacked him, but all the others saw were some irritated pigeons, so they ran away. Soon only Mortem and Catastroph (what he named the hellhound) were remaining. The Stymphalians swooped down, and Catastroph started barking at them. Noticing how the flinched at the sound, Mortem started screaming at them as well. Running more into the park, he went into the dark grove, where he tried to hide. The Stymphalians managed to find them, but by then Mortem had come up with a plan. He told Catastroph to start barking, and fairly quickly they were disorganised. Reaching through the world around him he made a small throwing dagger from the astral energies, which he hurled at a Stymphalian Bird that had stopped on a tree. It hit the monster head-on, and Mortem felt its life force get snuffed out. Feeling the rush of energy, he created more daggers and skewered the remaining four birds. He got Catastroph to go "fetch" the feathers and pocketed the ones that he rounded up.
His second attack was 6 months later, where a duo of fire-breathing horses attacked the orphanage. All of the occupants apart from Mortem and Max died in the resulting fire. The entire orphanage burnt down, destroying all the files and documents within. Max knew that he could now take Mortem as he was no longer an official ward of the state.
Max revealed to Mortem his true nature as a satyr. He showed Mortem to a small storeroom in the garden at the back of the orphanage which contained the items he had arrived with. He told Mortem to use whatever he could to open the book, and Mortem made a dagger which he tried to open it with. However, the moment the blade touched the lock the blade disappeared, and the lock opened. Mortem opened the book, and dark writing started to spread out from where he touched it. The first page filled in, and he read of how his mother had committed suicide, how his father was Thanatos, and how he was in danger and needed to leave to Camp Half Blood. The book also spoke of how he should put on the clothes that were next to the book. Mortem slipped them on, and found that they were the perfect size for him, even though he could have sworn they looked way bigger before. He also slipped on the watch. The book told him how the watch could turn into a scythe if he spoke the command word: "Skotono".
In shock by everything he had just encountered, he asked Max what they were going to do next. Max answered that they were to get to Camp Half Blood as soon as possible and that once there he could train and get stronger. That night he had a restless sleep where he dreamed he was being followed by a giant shimmering cloud made of Stymphalian Birds. When he woke up he spent 15 minutes readying himself, before getting out of bed. Slipping on the clothes and watch he went outside and met up with Max. They started walking...
From their orphanage in Oregon, they started walking to New York. Along the way they fought a monster every 6-8 days, even fighting (then running from) a Cyclops. Over the course of the 14 months it took to walk they fought almost 130 monsters in 60 different attacks. Unfortunately, they were attacked once more by a nest of Myrmekes just before they reached the Boundary. In the ensuing battle Max lost his life. Mortem blacked out after watching -and feeling- Max's death. He woke up in the Infirmary 2 days later, with a groggy headache and Catastroph by his side. 3 days later, he was still recovering.