Prize for Slytherin will be 24 hours early entry into Chamber of Secrets. They have not issued a date for when this is occuring, they did however state in the Pottermore Insider that only Slytherin accounts that existed in Slytherin at the time the cup was awarded will have access.
“As part of their reward for winning the Pottermore House Cup, Slytherin will be able to explore the new chapters of Harry Potter and the Chamber of Secrets, 24 hours ahead of the other houses.
Please be aware that only students who were in Slytherin when the inaugural Pottermore House Cup was awarded, will get early access.
For students who are not in Slytherin, don’t despair. The first chapters of Harry Potter and the Chamber of Secrets will be available for everyone immediately after this 24-hour period.
From then on, all users will be able to explore the new chapters, discover new Moments and continue their journey on Pottermore.
It’s official, as of around 9:00 am BST Slytherin has been named the victors for the Pottermore House Cup.
I’m sure a lot of you have come by to see if I have posted what the secret bonus is, as I am a Slytherin. They haven’t told us yet! Rest assured once I know I shall return to this entry and tell you all.
I am so very happy! Thank you to everyone who’s ever dueled me! Thank you to Gryffindor for making it a spirited fight since opening! Thank you to Ravenclaw for always being there to duel us! Thank you for Hufflepuff for never giving up, and earning points all the way to the end!
Here’s what Pottermore Insider had to say about the cup:
“We are delighted to announce that you can visit the Great Hall to view the award ceremony for the inaugural Pottermore House Cup.
Thank you to all houses for making this such a spirited contest.
We will be congratulating the winner in further detail, here on the Pottermore Insider tomorrow but in the meantime, good luck to all and enjoy the ceremony!”
So there is a chance we won’t find out about the secret bonus until tomorrow.