The golden egg
The egg, he decides, is a genius idea and a riddle he’s determined to solve before everyone else.
One day he opens it in the common room and all the merpeople near their underwater window (or wall? he is not entirely sure) turn to him.
He closes the egg, grinning widely. He loves riddles, mostly for the victorious feeling you get when you solve them (‘you’d do well in Ravenclaw, but you’d fit better elsewhere, I believe’). And he had a feeling he had just solved one.
A trip to the prefects’ bathroom later (courtesy of one Cedric Diggory) and his suspicions are confirmed.
Merpeople.
Which means the next task is gonna be in the lake (Estelle scoffs at that, muttering about idiots and hours of boredom because ‘how the hell are we supposed to see anything when you are down there?’)
(Cassius might have fallen just a bit more in love with her)
He doesn’t know if the other champions solved it.
He does know he owes Potter one.
So one day he catches up to him, Granger and the Weasley boy.
The other two immediately take a protective stance, but Potter just tenses minutely, green eyes studying him.
“The scream is a riddle. You need to listen to it underwater to understand it.”
Potter nods and he moves to walk away.
“Why should he trust you?”
The Weasley boy has guts, he’ll give him that.
“Don’t feel obliged to” he says “but now we’re even” he adds, looking at Potter.
Then he turns around and walks away, leaving three stunned fourth years in his wake.