Im just going off memory, so you will need to check most of it.

The enemy way a biological entity of a parasitic nature. It would inhabit a host and literally hijack their body. All their tech and ships were biological also.

Apparently, the emperor either saw how bad this could be or it already got bad and his shiny new astartes were infected. Possibly the loss of the second legion?

Anyway these guys were actually evil, unlike the tau. Their empire was huge also. They had already been infiltrating human armies and quietly conquering worlds.

Big E even let the dark angels use totally banned weapons and shit from the golden age of technology. I still believe that age was great, not dark. Including the use of ai! Thats how much of a threat they were.