It's a common misconception that wyvern eggs need to be kept hot or cold. The truth is, they can be kept at any temperature and will hatch just fine. It's actually the humidity that's important, not the temperature. Wyvern eggs need to be kept in an environment with high humidity, around 80%. If the humidity is too low, the egg will become dehydrated and won't hatch. Too high, and the egg could start growing mold which will kill the embryo inside.