A Living Poison.

Legion used a mundane virus as the base material for the new Legion-8's body. This virus was incredibly virulent. It could kill infected victims quickly as it proliferated rapidly within them. But this prevented it from spreading itself to other hosts fast enough before its current host died. 

The virus was genetically and magically engineered into a hive-like entity with a divine ability based on the law of poison. This made the virus sentient and improved its infectiousness. 

Then the hive was fashioned into a single body with the help of the Tree Father and then combined spiritually with the soul fragment of an Origin god that Legion-7 had stripped from Legion-1. 

The process was painful for Legion-1 because Legion-7 had to purge the soul fragment of his body and concept since he had completely become an Origin god.