Here is my Demon Twin AU. Please feel free to make this into a fic :) (if you do please let me know I would love to read it lol)
Damian al Ghul was never an only child.
He was born with a twin brother, Danyal. While Damian excelled in the League of Assassins, Danyal was born with a severe heart defect that left him too weak to endure the League's brutal training. Ra's al Ghul quickly deemed Damian the more worthy heir, treating Danyal as little more than a disappointment.
Despite this, the brothers were inseparable.
When they were only five years old, Ra's decided Danyal's weakness was a liability. To teach Damian that attachment was a weakness, he murdered Danyal in front of him.
Damian never forgave himself for failing to protect his brother.
Horrified by what her father had done, Talia secretly began planning Damian's escape from the League. When he turned ten, she delivered him to Bruce Wayne.
Although Bruce and the rest of the Batfamily tried to welcome him, Damian kept them at arm's length. He had already lost one brother. He refused to let himself have another.
A year later, after finally accepting the Batfamily as his own, Damian is struck by a magical curse during a mission. Constantine and Zatanna manage to save him, but while repairing the damage to his soul they discover something impossible.
Damian is part of a pair of Soulbound Twins.
Soulbound Twins are born sharing a single soul split into two equal halves. They are destined to be reborn together as twins in every lifetime. As long as one half exists, the other will always find its way back. However, if one soul is ever completely destroyed, the other will soon follow.
The Batfamily is confused. Damian has no twin.
Then Damian quietly asks the question that silences the room.
"Does... does this mean I'll get to see Danyal again?"
For the first time, Damian tells his family about the brother he lost.
They grieve not only for Damian, but for the little boy who should have been part of their family all along.
Unfortunately, fate isn't finished with them.
Three years later Talia creates a clone of Damian known as Heretic. Just as in canon, Heretic murders Damian during battle.
Bruce desperately tries to recover his son's body, but Darkseid claims it first, hoping to exploit the Chaos Shards hidden within Damian. When he realizes he cannot keep them, Darkseid destroys Damian's body entirely.
If Darkseid cannot have Damian, neither can Batman.
Bruce is left believing his son is gone forever.
But Soulbound Twins never remain separated.
In another universe, the brothers are born again.
This time they are Daniel "Danny" Fenton and his twin brother Dante "Dan" Fenton.
Danny is the reincarnation of Danyal.
Dan is the reincarnation of Damian.
Unlike their first life, the twins grow up together, always protecting one another.
When Danny accidentally activates his parents' Ghost Portal, Dan dives in after him without hesitation. Both boys die in the portal accident and awaken as half-human, half-ghost heroes.
Together they become known throughout Amity Park as The Phantom Twins.
Their circle grows to include Sam, Tucker, Jazz, Valerie, Wes, and Dani.
For the first time in any lifetime, the twins are truly happy.
Then everything falls apart.
After Jack and Maddie Fenton discover the twins' identities, they hand them over to the Guys in White. Danny and Dan are imprisoned and experimented on for months.
Their friends launch a rescue mission.
But every one of them pays with their life.
Valerie, Sam, Dani, and Wes die buying the twins enough time to escape. Jazz and Tucker stay behind long enough to activate the Ghost Zone portal one final time before destroying it, sacrificing themselves in the resulting explosion to prevent the GIW from ever following.
The Phantom Twins survive.
Broken, grieving, and barely alive, the twins are found by Clockwork, who brings them to the Far Frozen, where Frostbite and the Yetis nurse them back to health.
Once they recover, Clockwork reveals the truth.
Dan and Danny are not ordinary reincarnations. Their souls are incredibly young, having lived only one previous life before this one.
That life was as Damian and Danyal al Ghul.
Clockwork offers them a choice.
He can restore every memory of their first life and send them back to the family they left behind.
Or he can leave those memories buried forever.
Without hesitation, the twins choose to remember.
Danyal remembers the brother he lost.
Armed with every memory from both lives, the twins return to Gotham.
One year has passed since Damian's death.
Then two ghostly twelve-year-old boys walk into Gotham claiming the impossible.
One introduces himself as Damian.
The other smiles gently and says,
"Hello... my name is Danyal. It's nice to finally you."
Damian meeting Bruce age 10
Danny and Dan Death age 10
Danny and Dan reunite with Bruce age 12