Spawns of Satan

There was a legend, a myth, that says there were demon angel hybrid twins who were born on a full moon lunar eclipse they are cursed as the children of Satan. And they are born to wipe out every rogue. That is exactly what happened on December 15th, 1999 in Cansa, Massachusetts....
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"Ahhh!!" The soon-to-be mother screamed as she did her best to deliver her child. The faint cries of the beautiful newborn filled the room.
"There's another one coming!" The doctor exclaimed as she handed the infant to the new father. The man stood there shocked cradling the crying child in his arms, still red from just being born. Another cry is heard and the father's eyes brimmed with unshed tears of joy. 'I can't believe I'm a father now. Of twins!' He thought. He was so shocked in holding the child that he didn't even notice the limp, pale bloody bodies of his wife and pack doctor.
The boy and girl was ripped from his hands by the dark stranger. The last thing the father saw was the stranger holding his twin children in his arms like they were his own,"You can call me Satan." His husky voice whispered harshly.
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The twins grew up in care of the dark stranger. They are the legendary Spawns of Satan, taking on the title of their guardian as best assassin known to the supernatural world.
But, when on a mission to eliminate all rogues that were on the strongest pack in the country's territory, the soon-to-be Alpha and his sister find their mates. And it happens to be the deadly assassins.