Let’s see how long does it take you to notice the illusion in this poster below. It was originally used to promote computer game called Dragon Age II, but I managed to dig up this draft version lacking all the promotional text.
The billboard was first spotted by Chris Anderson, who decided to share it with us. He explained how he had seen this poster of a bus shelter couple of times before he finally spotted the negative-space illusion it hides. This is the best clue I can give you, except maybe what you find may represent all of the dragon’s victims ;)