It's Almost Like They're Made By the Same Company or Something...

Tin Foil Hat time folks; what if the demonic forces attacking Sanctuary are connected to the Burning Legion?

One of them are the result of betrayal against their own master, one of the 7 evils. But instead of killing them, the master instead attached that "thing" as an arm and filled the creatures with poison which can kill their enemies, yet not hurt the master. The other... Old gods.