GTA: Vault of Xenos
There was chaos a long time ago, but the Xenos came to put a stop to that. Since then, the world was at peace again and there are no need for the Xenos, so they have been sealed. However, evil forces are back again and now, you need to release one of the Xenos to stop them. Of course, you would need the other Xenos to do that, but for now, you only have one key. The idea here is to protect the post, you need to destroy the monster as fast as you can. You would start with just one Xeno, but as you move on, you would have another Xeno to control. If you have more than two Xenos then you need to choose which two would you like to have.