Fume Armors gradually slaughtered all the monsters that were in Fume Cathedral. They didn't need their energy and weren't defending their territory, they were just following their new lord's orders.
Gradually, Oskar, Erla, and the others realized what Plague Reaper's plan was. This monster was too arrogant to use the strength of others to achieve victory.
Usually, arrogance led to problems and failure, but... that was hardly a threat to Plague Reaper. He was too strong, a little help from Fume Armors or monsters was insignificant.
In fact... He could have killed all those monsters in a couple of seconds, considering they were locked in a closed space. He probably would have done the same with the Phantoms, but Osana's ability protected them, so he couldn't just spread black air and kill them like he had done with Fog Hunter earlier.
