At this point we all know that the Emperor can do a little brain invading and possess certain people. However he's not the only one in this stage of the timeline who can do something similar. For example, Killiks have this to some degree with a communal mind.
On a mid level planet, we're introduced to a certain intelligence that borg-ifies a number of non-force sensitives into its personal army, even referring to to his minions as his "hands". To me, it seems entirely possible that he can possess them to a degree and act directly. For whatever reason (no spoilers) this cybernetic threat hasn't made its way out to the entire galaxy.
This is as great a threat as anything we see in the game. Lets say an infected made its way to Korriban. Shortly thereafter, this intelligence could have an army of force users linked to its will. In addition, he can control his army directly (as opposed to a Sith Emperor that has to work through intermediaries). As a result, we have a singular malign intelligence which (potentially) can body hop.
If a Sith Lord could work this, he'd be more omnipresent than Vitiate.