Breach Hive removed Einhar beast

When I joined the area in act 8, I instantly got a "Mission failed" notification. Einhar quest says "Mission failed. The target was slain"
The only chat message from Einhar was the initial "Ready, exile? It is time to hunt!"

Since it is an act area, the mission could have spawned only one beast, which, most likely, was removed by the Hive.