#RPGaDay2016 day 10 - Largest in-game surprise you have experienced

GENCON 2012. Ubiquity/Hollow Earth Expedition adventure run by ReaperWolf. I pray this isn't a spoiler for anyone. Sorry if it is....

The big bad monsters we were just barely surviving up against... were babies! And the mother was waking!

Probably a pretty standard twist, but so masterfully done it blew my mind. Never forget it!