Okay so I wasn't planning to get on the DC x Danny Phantom crossover train but then I got the galaxy brain thought of 'hey, I never saw any fics where Danny's long lost Bat relative is Cass' and then it wouldn't leave me alone even though I haven't watched Danny Phantom in years. So here it is I guess.

Because of that, I'm gonna set this early on in the show where Danny would be still new at the whole ghost thing so he reasonably has very little knowledge of ghost culture, canon or fanon-wise.

DISCLAIMER: Don't own Batman or Danny Phantom. They belong to DC Comics and Butch Hartman respectively.

Haunted by an Unknown Past

By Bilbo-sama

The Fentons were in the lab discussing ideas on what to do for when you're in the house, unarmed, and a scary ghost attacks you when an actual ghost emerged from the portal in plain view.

"Ghost!" yelled Jack as he and Maddie quickly grabbed weapons and nudged Jazz and Danny behind them.

The ghost, who resembled an ordinary yet transparent human man, looked around in curiosity before spotting Danny peeking behind Maddie.

"Oh. You..." the ghost tilted his head in thought.

"We're armed, spook!" warned Jack as the bazooka in his hands warmed up.

"You sort of look like the little girl David Cain used to kill me," said the ghost.

"Uh," started a confused Danny who stepped into view, ignoring hissed instructions to go hide, "I'm a guy and fourteen? Also, uh, yikes dude."

"Indeed you are. I don't suppose any of you know where Cain is?"

"No idea who that is," shrugged Danny.

"Mom?" said Jazz who was looking at her parents. They seemed to be remembering something.

"That name sounds familiar," frowned Jack, "doesn't it, honey?"

Realization bloomed on Maddie, "Oh, that woman we met once mentioned him! But that was fourteen years ago! But why should we let that ghost," she spat the word like it was dirty, "roam free?"

"Because he'll finally be at rest after avenging himself?" offered Danny.

"Danny! Revenge isn't going to make the ghost feel better!" lectured Jazz.

"Whose to say he'll stop after one?" added Maddie.

Yelling erupted as the family debated amongst themselves. The ghost stood there for a moment before escaping the lab into the outside world.

A few minutes later, Danny noticed the problem.

"Guys! The ghost is gone!"

"Let him be," sighed Jack, "besides I'm realizing how big of an anti-ecto gun blast you dodged all those years ago, Danno."

"Wait, what?" Danny stared at his father.

Maddie gasped, "Jack, you mean...?"

"'I already got him what he wanted,' that's what she had told us before leaving," Jack sighed and looked up at Maddie. They stared at each other for a moment before reaching some sort of agreement.

"Do you remember when we told you that you're adopted, sweetie?" started Maddie.

"Yeah?" prompted Danny.

"Fourteen years ago we were approached by a woman holding a baby. She said she had no plans to raise a child and her goals and background don't align with child rearing," Maddie revealed. "Your father and I had been discussing having a second baby and then suddenly someone conveniently shows up the moment as soon as we finally agreed on going forward."

"Sounds too good to be true," said Jazz with a frown.

"We pressed her to explain and she said she was contracted to provide someone named David Cain a child. She had twins and he only wanted the girl. He had plans for that baby and she knew that if they failed then he would come for the boy next."

"So you wanted to spare me?" asked Danny, looking as though his world went upside down.

"We wanted you to be a child," Jack sighed, "and you got to be one unlike...your sister."

"That poor girl," Maddie shook her head, "we set that ghost on her."

"I don't think the ghost blames her, mom," Jazz pointed out, "she wasn't at fault. Her dad is."

"Well uh," started Danny before he hugged Maddie and then Jack, "thanks for letting me be a kid."

"No problemo, Danno!" grins Jack before hugging his son tightly, hearing him squeak out a "Too tight!" protest. "Now where were we on those ghost drill plans? I got some more ideas!"

I have no plans in continuing this plot but I do have this image of our new ghost buddy coming across Cass along the way in his hunt for David Cain and somehow gets her under Batman's radar early. And this in turn has Cass join the Waynes ahead of schedule and long story short Jason doesn't die.

Deus Ex Machina Ghost returns to the Fentons and is like 'good news, she escaped and is now a rich guy's feral daughter do you want their number?' And then he just. Pops up time to time like a sitcom neighbor.

He's forever on Clockwork's shit list.