I'm not from this age: Future origin
The origin of Dagon starts 10 years into the future. In this timeline the villain known as Lord Chaos reignes the world with tyranny. Dagon is one member of the team known as the "Team Titans". This titans were created to defeat Lord Chaos and restore the peace and freedom of the world. This timeline was proven false.
David was born in London, he was raised there and lived in harmony there for 10 years, until one Christmas Eve. That day was a dark one for Davis. The first reason was that he had been strucked by a truck, the other one was that just after that he had been taken in by Lord Chaos. Lord Chaos made his scientists experiment and inverstigate with Davis's body. However Davis was able to escape after those experiments. After that he started to aid the resistance on certain occasions.
The years passed, and when he was fourteen years old, he was taken to the country of Transylvania to have a new and unused experiment. Apparently the scientist known as Commander Stalg (One of Lord Chaos's scientists) had been able to get the bones of the most important vampire of them all, the Count Dracula. The master plan of Stalg was to extract Dracula's DNA from his bones and infuse it into David. Along Lord Chaos's soldiers was one named Charlie Watkins. He had always been a loyal sevant to Lord Chaos's, however when he saw the experiments that Stalg conducted on Davis he started to consider if Lord Chaos's way was right for doing this to a human that was just a child.
Stalg's experiments succeeded and he was able to infuse David with the DNA of Count Dracula. However something unexpected happened. A demon was angered because of this experiment, so he entered David's body during his tranformation, changing the result of things. That night David became a vampire, however he also had an afinity to the demon. After that David started an attack against the scientists, he called himself "blood-hungering dagon". After the fight ended he would call himself Dagon. He also realized that David was now dead and that he was now only the vampire known as "Dagon". Dagon killed almost all of the soldiers of Lord Chaos that were in that place, with the exception of Charlie Watkins. Soon after that he joined the resistance and later the teen team known as the Team Titans.
One day the leader of the Team Titans decided to make one risky and dangerous mission. The team titans would travel 10 years into the past to kill Donna Troy in an attempt to stop Lord Chaos from being born. Of course this had to be made before Donna Troy was able to give birth to her son (The future Lord Chaos).
The first part of the mission was succcesfull and the team of the future (Team Titans) were able to go back to the past before Donna Troy gave birth to Robert Long.
When the Team Titans tried to kill Donna Troy they encountered the team of Titans from the present (the new Teen Titans) and fought them.
However before the Team Titans could kill Donna Troy she gave birth to her son. Altough the mission looked to have failed the Teen Titans and the Team Titans were able to both stop the future of Lord Chaos and prevent the death of Donna Troy.
We did it...now what?
After the mission was acomplished the Team Titans were trapped into the past, with no way to return home. Without any options they decided that they would stay on that era until they could find a way to go back. In an ironic twist they accepted the offer of Donna Troy to live in her farmhouse in the city of New Jersey. After that they decided to investigate this timeline and find where they belonged in it.
Erased from history
Zero Hour started, changing the Team Titans in a way that no one could have forseen.
In this time crisis was revealed that the real leader of the Team Titans was the being known as Monarch (Previously known as Hawk). The Team Titans were actually beings that were from a false future created by Monarch, who wanted to have sleeper agents that would serve him as assassins in order to help him to rule the entire timeline. That future also had another purpose; it would help Monach to train young metahumans and have a bigger and more powerful army against the heroes of all generations.
Monarch (Now as Extant) absorbed the power of Waverider and used it to take adventage of the incoming time crisis. Using the army that he created, along with the Team Titans he attacked the world's heroes who wanted to stop the time crisis.
However Extant's plan was unsuccessful and the Team Titans were defeated by the Earth's heroes. At the same moment, time started to collapse. This made all the parallel and false timelines to be erased from existence. As another result the Team Titans were erased from history and dissapeared. The only three team titans members that weren't eased were Terra II, Mirage and Deathwing because it was revealed that they were from the present timeline and that Extant had taken them from there, erasing their memories to make them part of his army. Dagon didn't have the same luck, and suffered the same fate as the other members; erased from history, no one would remember him because he never truly existed.
It hasn't been stated if a Dagon from the present will appear in the DCU, and only time will tell