Ion!Potter

Harry fingered the wad of slimy weed then stuffed it in his mouth at the sound of the cannon.

Pain began to wrack him and he leapt into the water. The cool water flowing over his gills—gills! He marveled—let oxygen into his body once more.

He began swimming, heading for the deepest part of the lake, darting away from grindylows, stunned at just how well he could swim thanks to the gillyweed.

He was peering around, looking for signs of his friend when a massive wave of air came rushing towards him. A memory of a submarine movie he had overheard came to him and he realize he was about to die.

He pushed his fear down and started swimming for the surface, knowing there was no way to make it in time.

Something seemed to speak in his mind. "You have the ability to overcome great fear!"

He spun around, trying to see what spoke and saw a weird slug like thing of green light touching then he felt a ring form on his finger as jade green light began to coalesce around the expanding shockwave from the asteroid that had trapped Ion.

He blinked, realizing he knew the history of a three billion year long organization destroyed in part by their own hubris.

"You are the last Green Lantern," Ion said in his mind as Harry said, "We—we're the last Green Lantern: Ion!"

The ring began scanning for lifeforms and pointed him towards four in suspended animation surrounded by eight score of merpeople.

Harry's swimshorts and tank top turned into a black jumpsuit with a green lightning bolt on his right leg looking like a tribal tattoo and a green stylized lantern over a heptagon on his right side of his chest with a dark green set of overrobes, the stylized lantern on his back in black with a thin white outline.