List of people Percy Jackson has scared the shit out of:
Clarisse, daughter of Ares, Drakon Slayer: “Believe me, revenge is coming. One of these days, he’s going to be sorry. Why am I waiting? Just strategy. Biding my time and waiting for the right moment to strike. I am not scared, okay? Anybody says different, I’ll rearrange their dental work.” - Demigod Files (Interview with Clarisse LaRue, Daughter of Ares)
Luke Castellan: “‘Luke feared you,’ the Titan’s voice said. ‘His jealously and hatred have been powerful tools. It has kept him obedient. For that I thank you.’” -BotL
Kronos himself: “"Perseus Jackson,“ one of them said. “Yes,” mused another. “I do not see why he is a threat.” “Who said I was a threat?” The first Hesperid glanced behind her, toward the top of the mountain. “They fear thee. They are unhappy that this one has not yet killed thee.” She pointed at Thalia.” -TTC
Leneus, a member of The Council of Cloven Elders: ”I didn’t push him very hard, but he was kind of top-heavy. He fell on his furry rump, then scrambled to his hooves and ran away with his belly jiggling.” - TLO
Hades, Lord of the Underworld: “Hades swallowed. ‘Now, Jackson, listen here…’ He was immortal. There was no way I could kill him, but gods can be wounded.” -TLO
Phobos, the God of Fear himself: “I couldn’t kill him. He was immortal. But you wouldn’t have known that from his expression. The fear god looked afraid.” -Demigod Files (Percy Jackson and the Stolen Chariot)
Leo Valdez: “Leo’s legs trembled. The way Percy looked at him made him feel the same as when Jason summoned lightning. Leo’s skin tingled, and every instinct in his body screamed, Duck!” -MoA
Piper McLean: “Piper guessed that Percy hadn’t meant to cause so much damage, but his glowering expression made her want to leave the ship as soon as possible.” -BoO
fucking Annabeth Chase: “Since she’d come back from Tartarus, Annabeth had told Piper about a lot of scary things that had happened down there. At the top of her list: Percy controlling a tide of poison and suffocating the goddess Akhlys.”- BoO
And let’s not forget:
How Percy looks when he fights: “My expression in the picture was fierce—disturbing, even—so it was hard to tell if I was the good guy or the bad guy, but Rachel said I’d looked just like that after the battle.” - TLO
Hazel’s first impression of him: “Then she’d met Percy. At first, when she saw him stumbling up the highway with the old lady in his arms, Hazel had thought he might be a god in disguise. Even though he was beat up, dirty, and stooped with exhaustion, he’d had an aura of power. He had the good looks of a Roman god, with sea-green eyes and wind blown black hair.”
Hephaestus’s words to him in BotL: “The god grunted. “Not that powerful, eh? Could have fooled me. You’re the son of the Earthshaker, lad. You don’t know your own strength.””
“Nico twisted his silver skull ring. “Percy is the most powerful demigod I’ve ever met. No offense to you guys, but it’s true. If anybody can survive, he will, especially if he’s got Annabeth at his side.“ -MoA
“Part of their problem was Percy. He fought like a demon, whirling through the defenders’ ranks in a completely unorthodox style, rolling under their feet, slashing with his sword instead of stabbing like a Roman would, whacking campers with the flat of his blade, and generally causing mass panic.” -SoN