Okay we know that there are five coffins in Mystic Falls four of which happen to host Klaus's brothers and sister so who is in the forth coffin? For some reason Klaus had this coffin sealed with magic to keep what ever is in side stay inside. So who could it be in that coffin that Klaus could be scared off? We know it's not Michael so could it be another original witch aka mom? We know that Klaus killed her but she was a witch and what if there was some sort of black magic that can bring her back to life clearly Klaus would not only what to keep mommy near by but make sure she never wakes up from her deathly sleep.

me too think what his mother in the 4th coffin...

She's Back Mom is Back!