During the Inferno, the Hobgoblin wanted more power, so he struck a deal with the demon N'astirh to imbue him with the power he sought. N'astirh agreed to his request and gave the Hobgoblin the power of a demon to add to his own might. However, the demonic power given to the Hobgoblin grew and began to take over the human which was Jason Macendale.
The demonic entity which would come to be Demogoblin first appeared in The Spectacular Spider-Man #147 by Gerry Conway and Sal Buscema (Still only considered as Hobgoblin though). The first appearance of the entity officially known as Demogoblin was Web of Spiderman #48 by Alex Saviuk and Howard Mackie.
When the Hobgoblin fought Doctor Strange, he seperated the demon from the man and inadvertently gave birth to the Demogoblin.
After that, The Demogoblin went out on his own. He started a mission to destroy all sinners in the world. He stated that the demon who possessed him belong to a group of demons calling themselves the "Righteous", they seek redemption by destroying evil wherever they find one. In most of his missions, he would often run into Macendale and try effectively to kill him.
In one fight with him, he fought Jason while Spider-Man was fighting Doppelganger. During the fight, Demogoblin was blasted by Hobgoblin's new gun and shot into the distance. When he got back up, he saw both Jason aka Hobgoblin and Spider-Man were gone. But he found a wounded Doppelganger, and decided to have him fight along side him.
A few of his adventures included him teaming up with Doppelganger. They had an uneasy alliance. He would often call Doppelganger Mindless One.
Infinity War: Demogoblin first met the Doppleganger, a creation of Magus, when the two joined forces against their counterparts, Hobgoblin and Spider-Man, during the confrontation, Doppleganger was impaled on a fence, but was brought back to life by Demogoblin sometime later.
Demogoblin and Doppelganger would be captured by the Death Spawn during a time when Hag and Troll tried to bring Deathwatch back to life. Hag and Troll were defeated by Ghost Rider, Johnny Blaze, Spider-Man, and Venom. Demogoblin and Doppelganger escaped.
He then played a big part in the Midnight Massacre when Blade was turned into Switchblade after being possessed by the Darkhold: Book of Sins. Demogoblin wanted to team-up with the powerful Switchblade, but was killed by Switchblade and inherited Demogoblin's powers. He was later brought back to life after Blade was cured.
Demogoblin then joined Carnage and his family during Maximum Carnage. In Venom's storyline "Enemy Within" Demogoblin fought Venom and Morbius in San Francisco during an attack of goblins infecting the city. Demogoblin as thought to be the cause of the goblins but it was really the work of a guy named Charlie. In the end Demogoblin killed all the other goblins by forcing them to jump of a bridge and drown to death. Throughout all of his violent perils over the years, this is probably the most heroic of his plans to stop sinners.
Demogoblin met his end after Hobgoblin threw a building on him. Before Demogoblin died, he rescued a kid from the falling building. Demogoblin died an unlikely hero in the process.
Demogoblin's spirit once tried to possess Moon Knight after he confronted the Hobgoblin. His spirit was then exorcised with the help of Dr. Strange and Mr. Fantastic.
Demogoblin also came to Phil Urich in a dream warning him about the curse of taking up the Goblin Legacy, it is unclear if this was Demogoblin's spirit attempting to warn Phil, or just Phil's own imagination.
Demogoblin possessed superhuman strength sufficient to lift/press ten tons.
Goblin Glider: Demogoblin flies on a fiery version of the Hobgoblins glider
Fire Blast: Demogoblin can fire blasts of fire from his fingers
Pumpkin Bombs Black: Demogoblin can throw a black pumpkin bomb which will produce a darkforce net which completely encompasses its target and shrouds it in cold and darkness, effectively stunning them.
Pumpkin Bombs Orange: Explosive bombs that cause massive damage.
Demogoblin thinks everyone is evil, and therefore must pay for their "sins" with their lives. This means killing anyone and everyone he can to further his cause. By his own admission, there was only one person he could ever bring himself to forgive, Jason Macendale, the person who brought him to this world.