Shadows Angelus began when I viewed the anime Silent Möbius for the first time (the Streamline release of the first movie, dubbed in English, if you care)—the story of a group of police officers with special abilities battling an invasion of demonic creatures from some black dimension somehow profoundly touched me, inspired me, and gave me a vision of a setting where I could tell the same kind of story in a different way.
There's plenty of information on the Silent Möbius universe here, but Silent Möbius was only the beginning. As the years went on, I gained a new appreciation for the Action Horror genre and added new titles to my mental "toy box" where ideas for Shadows Angelus continued to grow.
Movies alone took me a long ways, but literature would add a spice all its own. The works of Stephen King, George R.R. Martin, and H.P. Lovecraft tumbled into the toy box as well, until I felt I was ready.
Here's a list of the most influential films, anime, and books that make Shadows Angelus what it is today:
Film
Big Trouble in Little China
Escape from New York
In the Mouth of Madness
John Carpenter's The Thing
The Robocop Trilogy
The Robocop TV series
Anime
Bubblegum Crisis (original series)
Silent Möbius (Streamline release of the first movie)
Silent Möbius (TV series)
Books
Everything's Eventual, Tales From A Buick 8, and Nightmares and Dreamscapes by Stephen King.
The collected works of H.P. Lovecraft.
The Song of Ice and Fire by George R.R. Martin.
The Warhammer 40,000 universe by Games Workshop (too many titles to list).