Right it may seem stupid but when Kai Parker (Malachai) was trapped in the prison world by the Gemini coven, why didn't he siphon the world away. This would either have resulted in him dying, as he tried to do, or he would come back to the real world. The prison world is created by magic and a siphon is able to get rid of previously cast spells, as shown when he removes the anti-magic border around Mystic Falls. He, and the heretics from the other prison world, would surely have thought to get rid of the prison, even if this ended with their death.
