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It's not that strict of a theme. It's more just like, use it as inspiration and if it goes off the wall a bit then that's fine. The word "Sacrifice" could be taken in many other ways, for example you could use "Halloween" elements (pumpkins, etc), but then you could interpret the sacrifice as something other than like a ritual sacrifice. With that in mind you could quite easily use the Halloween theme and make it fun, bright, etc. I don't think anybody is going to score you low because it's not scary or doesn't have a ritual scene. It's really just a springboard to get you going.

"Halloween" is a super rough guideline because we didn't want to restrict people to just "Horror" or "Ghost" or anything like that. You can theme the levels with halloween themed creatures and assets, for example, or just have the main character as a ghost. Pretty much anything you yourself consider to be Halloween-themed can work. Also there's a rule which says the game doesn't have to be horror and that we want to encourage people to be creative, so I have a feeling that the majority of the games won't be horror. Hopefully this helped and good luck on your game!