What are unlockables for Eragon?

Answer 1

There is a secret egg (looks like Saphira's egg) on each level (including the Saphira and Eragon levels), which unlock a video about an aspect of the game. Eggs do not need to be collected in order, though the videos will be made available in the same sequence each time.

Once all the eggs have been collected, a Survivor type level is unlocked, where Eragon and Murtagh are in the Throne Room, and fend off never-ending waves of enemies until the player gets tired of playing or one of the characters succumb to the enemies.