Darius Dun  is the late boss and financier of a local New York City gang called the Street Phantoms in the dimension occupied by the Teenage Mutant Ninja Turtles.
Teenage Mutant Ninja Turtles
During Splinter's reign of the Foot Clan, they came into conflict with the Street Phantoms. Ultimately, Splinter defeated Darius Dun in a duel but he refused to bow and surrender his assets. As a result, he was executed. The Turtles were appalled and their relationship with Splinter became rocky. For more information, see here.
In the afterlife, Darius met Bronson, a welcoming spirit who tries to help other ghosts become acclimated to an existence in the ghostly limbo. Bronson taught Darius how to open a window and observe the physical plane but he soon came to regret that decision. Darius still held onto his grudge against Splinter for stealing his legacy. Darius was enraged to see the Foot Clan had taken over half of New York City. He vowed he would go to any length to even the score. Bronson recalled that for being around for awhile, he heard of beings who could travel through dimensions at will and only existed to hunt and capture the living. Darius cut him off and asked how to contact them. Bronson warned him against rushing into a deal because the cost to summoning them was awful, even for a ghost. Darius didn't care and asked again. The Collectors were summoned and sent after the Teenage Mutant Ninja Turtles, Splinter's sons. Darius wanted to strike at Splinter where it hurt most, his heart. Then Darius would watch him grow old and desperate, writhe and suffer, then deny him mercy and make him continue to suffer. The Collectors captured Leonardo, Michelangelo, and Raphael initially and imprisoned them in their prison dimension then went after Donatello. Leonardo wanted everyone to remain calm so they could get their bearings, chart the territory, and start working on an escape plan. Darius manifested as a skull and told them there wasn't a way out. Darius turned into his normal form and promised to break them. Michelangelo had no idea who he was. After referencing his refusal to bow to Splinter, Leonardo realized he was Darius. He admitted there were some definite advantages to being a ghost then revealed his plan to them. Darius promised he might release them after Splinter died or perhaps not. Darius laughed and returned to the ghostly limbo.
Darius decided to peer into the Collectors' Limbo and check in on the Turtles but realized they were gone. He became infuriated and demanded to know how they escaped. Bronson tried to calm him down and warned him the agitation was the start of the Collectors' cost of summoning them. Darius didn't like being patronized. Bronson denied it and warned intensely negative vibes had more consequences in the limbo than on the physical plane then assured him the Collectors were likely already searching for the Turtles. Darius summoned the Collectors back to demand an explanation for their failure. Bronson was alarmed. The Collectors were angered he cost them the chance to capture Leonardo. Bronson tried to step in and cover for Darius. He wasn't having it and shoved Bronson aside. He stated he expected results from those he hired. They found Darius mad to summon them again and decided to take the entirety of his being than a piece of his soul. They were poisoned by the growing madness in his essence and were completely taken over by Darius. Bronson was horrified at what he caused for trying to be a friend. Darius didn't feel the same way. He felt "reborn" with his mind completely restored and possessed all of the Collectors' knowledge and power. He realized was no longer bound by their rules and could travel wherever he pleased. He decided to collect the heads of the Turtles then kill Splinter.
Darius sensed the Turtles in the Ghostbusters' dimension and teleported to Central Park. Peter Venkman addressed them and played his part in stalling them until the Turtles got into position and attacked. Peter offered him the chance to surrender. Darius was amused but realized there was a condition in place that required him to deal with the Turtles first before he was completely free to do whatever he wanted. Darius revealed he planned on killing the Ghostbusters and Turtles then he would build an eternal criminal empire across the multiverse. Peter found him "full-tilt bugnuts." Leonardo sliced the green Collector in half at the torso with his Proton Swords. They were surprised to feel pain but vowed to destroy them. The Turtles and Darius fought. The Turtles acknowledged none of them voted for his execution but his threats forced Splinter's hand. Darius found their words irrelevant and vowed to show everyone with his new power. He stated their Proton Weapons couldn't defeat him and the more familiar he became with his new forms, the more his power would grow. They failed to notice Ray Stantz and Egon Spengler fine-tune the Trap-Gate to their unique P.K.E. signature. Peter and Winston came to the Turtles' aid and opened fire. Darius recalled the Collectors could not be contained by the Ghostbusters and underestimated the Trap-Gate. He vowed to escape. Egon pointed out it wasn't like the Traps. All four were drawn through the Trap-Gate's portal into the Containment Unit.
- While driving from western Minnesota to Fargo, Erik Burnham talked to co-writer Tom Waltz on the phone about picking what villains to use. Shredder came up as a potential but Waltz wasn't interested and almost immediately suggested Darius Dun. 
- Cover B of Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #1 features Darius Dun.
- Darius' bio in Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #1 first refers to his death at the hands of Splinter and the Foot Clan. This took place in Teenage Mutant Ninja Turtles #64.
- On page 12 of Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #5, Darius Dun alludes to his murder in Teenage Mutant Ninja Turtles #64.
- On page 15 of Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #5, Donatello, Leonardo, and Raphael refer to the circumstances of Dun's death.
- IDW Comics
- Leonardo (2017). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #1" (2017) (Comic p.17). Leonardo says: "That's -- it's Darius Dun!"
- Down and Nerdy Tweet/Episode 188 47:23-48:02 11/10/17 Erik Burnham says: "You know this all this -- all come picking the bad guys -- all came up, I was bugging Tom while I was on a long and very boring drive across western Minnesota where there's a not a lot to see up to Fargo. So I - so I was just keeping him going so he would stay on the phone so I wouldn't have to deal with the landscape. But uh no we were coming up -- we were throwing out all kinds of ideas. Shredder uh as he is dead right now y'know a couple spoilers from a couple years ago. He came up as a potential because hey now he's a ghost now he can do some stuff. Maybe he does this, maybe he does that. Tom didn't to want to go there, came up with Darius really fast, I mean immediately."
- Narrator (2017). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #2" (2017) (Comic What Came Before Page). Narrator says: "DARIUS DUN, the crime lord assassinated on the order of Splinter, is now a ghost on an interdimensional limbo."
- Leonardo (2017). IDW Comics- "Teenage Mutant Ninja Turtles/Ghostbusters Volume 2 Issue #2" (2017) (Comic p.17). Leonardo says: "Dun's after us because my father had him assassinated."