What is the True Ending in Hogwarts Legacy after you've completed the main storyline?
The House Cup is the second main quest post-game within Hogwarts Legacy and acts as the game's True Ending; where you attend the House Cup Ceremony in the Great Hall, and see if your House is the victor!
To achieve the true ending of Hogwarts Legacy, the player needs to complete The House Cup main quest and win the House Cup award at the end of the year. The House Cup quest becomes available at level 34 and is the true final chapter of the game.
The true ending includes a scene in the Great Hall where the player's house wins the House Cup and a conversation with fellow student Sebastian Sallow.
You can read more about it here and watch a video.