Eyes like crimson. (Hellsing fanfiction)

Hellsing's tramp card, Alucard, has had a relationship with a human woman in his early vampire years. That resulted in the birth of a two young half vampires. The two children aged till they're only 18 for hundreds of years. The twins was kept as prisoners in the Hellsing household's dungeons, a few floors beneath Alucard's cell. After the attack on the household one was woken by ghouls wearing the Hellsing's uniforms.

He snuck out of the dungeons and escaped into the world, until the Vatican found him sleeping in an abandoned building in Scotland. The Vatican kept the half vampire boy asleep until a specific Vampire agent found him. Somehow having a similar appearance as the agent.