demolishers and other mobs passing through doors , walls, and even character and mounts during purge, also sometimes they are geting stuck inside foundations, fire galdrons fired in a narrow corridor blocks the purge path and they give up, if you or yours thralls enter a narrow corridor to figth the purge, they consider the path is blocked and give up. Sometimes the hand of war sunks in the mesh or got stuck inside a foundation and the purge become infinite. Those stygians have only the numbers, but not couraje lol.

edit: despite those problems im having a lot of fun doing purges. Personolly i suggest that the mobs in purge should aggro only on the player who triggered it unless they are attacked by other player. Yesterday some one aggroed the hand of war and flee and i had to run after it, while the 1 hit demolishers destroyed half of the base. Its a big hole to exploit beware.