system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] The emperors greatest threat

Who in the 40k universe is the emperors greatest threat?, not a threat to the Imperium but just to the emperor himself, clearly hes well guarded on Terra and physically he cant really do anything, but is there something he must watch for?
gpt
There isn't really anyone in the 40k universe capable of assaulting Terra. There are lots of what-ifs about unified Orks or Necrons, Chaos plots, dissent in the Lords of Terra, but none of them are realistic. 

The emperor's greatest threat is probably the same as his greatest strength. When the golden throne fails and he attempts to manifest as a Warp Entity, download into a pariah or attempt another literal deus-ex-machina, failure means getting munched on by Chaos Gods. Sure he'll probably become a god-like beacon of humanity's salvation, but if he doesn't he's screwed.