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.