Phobos: Greek God of fear
Sirius stands in front of a door. According to the map, there's no one on the other side, but he can hear banging coming from within. He spells the door unlocked and pushes it open carefully. Lighting the room so he can see, it appears empty. But then the chest in the corner of the room shakes violently.
He casts a spell to open it from where he's standing, not wanting to risk getting any closer. The lid of the chest throws open and the door behind him automatically slams shut. So much for leaving, he thinks to himself.
His eyes widen in shock as he watches himself climb out of the chest. That's impossible. For a few moments, he stares at himself, taking in every detail. Then he realizes what it is. A Boggart. Chuckling silently, he wonders why he would be his own worse fear.
"You're just like them," the Boggart taunts. "You can't escape your destiny. You'll always be like them."
Even though he knows better, Sirius takes the bait. "No, I'm not," he says through gritted teeth. "And I never will be."
"You are, though. You have their blood running through your veins. You're just like them," the Boggart-Sirius repeats. "Always loyal to people you care about. You're forgetting that it's a Black family trait. Just like you enjoy the popularity and power you get. Blacks have been drunk off that pleasure for centuries now, and you're no different."
Sirius' hands ball into a fist as he tries to control his anger. It's not true. It's not true, he chants to himself.
It's like the Boggart can read his mind. "But it is and you know it. Or you wouldn't try so hard to be different, to stand out. The problem is that you can never escape the darkness inside you, no matter how hard you try. It'll always be there. It'll always be waiting in the shadows, trying to consume you."
"It won't consume me," Sirius states. "I won't let it."
Boggart-Sirius smirks. "It has already. You left your own brother in the hands of your family. What kind of monster would do that?"
Sirius shakes his head. "I offered to take him. But he wouldn't go with me. He was terrified that if our parents found him with me that they would hurt me," he says in exasperation. "I wanted to save him. I really did."
"You can keep telling yourself that, but we both know it's not true. You wanted him to experience the same pain and torture you did," the Boggart replies easily. "And that's what your ancestors have been doing since the beginning: forcing others to feel their pain."
That's not true. It's lying, Sirius chants to himself again.
"And how did your friends forgive you for leading someone into a certain death?"
Sirius looks up, meeting the Boggart's gray eyes. "I didn't mean to. I was just so angry."
"James compared you to your family, didn't he? He said you're exactly like them," the Boggart says. "He's right; you are exactly like them. Blacks can't control their tempers and neither can you. It's in your blood."
For the first time, Sirius finally remembers the spell to vanish the Boggart. He pulls out his wand and points it at the duplicate. Thinking of the funniest thing he can, he whispers, "Riddikulus."
But the Boggart doesn't change. "You can pretend to be anything you want, but you will always bleed Black."
Tuning the Boggart-Sirius out, he chants, "Riddikulus," several more times. When he finally opens his eyes, he starts laughing at the scene in front of him. He charms the Boggart back in the chest and leaves the room as quickly as he can.
A/n – this is set sometime during Sirius' sixth or seventh year.
A bunch of thanks to Budapest All Over Again for beta-ing this.