(Okay, so I found one more…)
Names to Run Away From Really Fast: In the Imperial context alone, Imperial Hand, Fifth Directorate, and any Imperial military officer, agent, or private contractor whose House name is “Sargas” are the chief contenders. In the Worlds as a whole, Operatives of the Conclave are also not to be trifled with.
The Photonic Network‘s OPSEC, the Voniensa Republic‘s Exception Management Group, and the Eilish High Guard are also notable in this field, and while most people wouldn’t rate them against first-rank regulars, Kestal’s Raiders have achieved a certain bloody success in the unbonded mercenary business, operating out of the Rim Free Zone.
At least some of these can also be Names to Trust Immediately. All depends on who you are, and who they are.
Also, as was mentioned under Overly Long Name, one of the traditional components of eldraeic names is the attributive name, based on your personal attributes and/or accomplishments, which grow increasingly significant as your reputation grows, up until you reach the people with really towering reputations who can introduce themselves only by attributive name, Exalted-style, such as Exquisite Engineer of Worlds, or Manyfold Propagator of Celestial Wealth. Of course, those aren’t terribly intimidating in this trope’s sense, but should you encounter, say, Bloody-Handed Avenger of Iniquities… start running.
(Curiously enough, despite their taste for lush verbosity, this effect only increases as the attributive name dwindles towards Gallifreyan sparseness. Anyone who could pull off simply introducing himself as “the Warrior”, for example, would surely be someone able to win a major fleet action armed only with a cheese knife.)