Once you've finished playing A Crow in Hell in Story Mode, there are three challenges you can play - if you've collected enough keys, that is.
(Didn't collect enough keys? Watch my Story Mode walkthrough - "24 Keys in 12 Minutes" here: • A Crow in Hell Walkthrough - 24 keys in 12... )
The second challenge is Falling Eggs (16 keys required), where you have to survive for two minutes without being hit by a falling egg (or, as usual, touching the ceiling or floor). It starts out easy, but gets pretty hard by the last ten seconds!
By the way, I don't know why the menu never crosses out this challenge after it's completed. Programming glitch, I guess, but it's a bit annoying.
Other Challenges
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Challenge #1 (Croa Croa Race) - • A Crow in Hell Challenge #1 - Croa Croa Race
Challenge #3 (Big Papa) - • A Crow in Hell Challenge #3 - Big Papa