It's the level where you must capture and convert the three princes, however as soon as I kill one and my imp starts to drag him to my prison the game finishes and tells me that I have completed the level, then it goes to the level select screen and my only option is to start that level again. It happens no matter which prince I attack first and have tried doing the level from a fresh a few different times.
Anyone else had this problem? Or a solution?

When you attack any of the princes, the others are alerted and try to flee to the nearest hero portal. You must not let them see you and try to block their path to the portals. It requires a lot of stealth to get through. Good luck!