I got this Idea from the fanfic 'Scared Percy Guardian of the Hunt' by GrayNeko. It's a wonderful story I hope you read the story if you'd like.
Four years, four years since his mother married Gabe. What his mother didn't know that while she was out, her son would be beaten by Gabe with whatever he had in hand. Once Percy had gotten use to the pain and whatever method Gave would switch it up.
"Remember twit, you talk. Your mother gets it." Gabe kicks Percy in the stomach and left, laughing filled with malice. Thirty minutes later Gabe was asleep when Sally came home. When she came to Percys' room she was horrified.
"Percy what happened? Who did this to you?" Sally looked at her son with fear in his eyes. He shook his head vigorously, afraid of his mother getting the same punishment.
"Gabe, he's been doing this to me ever since you married him, and now he's going to do to you wat he did to me." Percy cried into his mother's shirt, not knowing that Gabe was standing in the doorway. What the hell are you doing runt? I thought I told you not to talk." Gabe pushed Sally aside, grabbed Percy by the collar of his shirt slammed him into the nearest wall, and began punching the young boy.
"What do I do? My baby boy has been beaten by this monster." Sally thought as tears threatened to escape from her eyes, not knowing there was a fourth presence until she heard a voice. "Put my son down you disgraceful mortal!"
"P-Poseidon?!" Sally gasped at the sight of the sea god of Greece mythology. "Who the hell are you? How did you get in the apartment?!" Gabe yelled at the man in Bermuda shorts and Hawaiian shirt.
Poseidon released a bit of his power and the earth around the apartment shook, making Gabe loosen his grip, allowing Percy to fall and run to his mother. Poseidon summoned his trident, and threw it at Gabe, not skewering him in the neck, but the tips cutting into the sides of his necks between the now filled gape. "W-who are you?" Gabe whimpered, peeing his pants.
"I am Poseidon, Lord of the sea, and I'm here to personally send your tainted soul to the Underworld for what you've done to my son. Should I feed you to a shiver of sharks, drop you into the Sea of Monsters, or..." Poseidon looked to his son and mother and nodded his head as his sea green eyes began to glow as the ground shook, hurricanes formed on the coastal cities, and the seas capsized ships. Sally, getting the message, had her hand on the back of her sons head and pressed him against her bosom and closed her eyes.
"You should considered yourself honored you filthy sum of a mortal, besides the Gods, no one has seen my true form. I'll make sure my brother sends you to the Fields of Punishment instead of the Fields of Asphodel. I'm sure Cerberus will be drooling over your foul soul." Gabe was shaking with fear as the Gods body began to glow. Percy could hear Gabe screaming at the top of his lungs until it grew quiet.
"Can we look now mom?" A young Percy asked as the glowing died down. "Yes, you can my child" Poseidon said, retracting his weapon as his son and mother opened their eyes.
"Thank you for saving our son Poseidon." Sally cried tears of joy as the God of the sea knelt down and petted his son as Percy looked into the man's eyes. "I'm sorry for taking away the only thing keeping my boy safe from the monsters…." He said as he turned to his son "Hello Percy, I'm your father."
I thought about making Gabe a Roman demigod but I decided against it. I hope you liked this story.