@puddleduck It seems that Ostravia is the only way to get through that door. He gives you, or drops, the Mausoleum Key, which is the only way to get through that door. Once he's gone, the key is lost for the rest of the play through. That's what my frustrated google searches were showing late last night! 
XBL: StefJam
PSN/Vita/Steam: StefJamEX
