The Ambassador Imperialis is a colloquial (or possibly not colloquial) name for the organization(s) within the Imperium of Man responsible for the legal and above-board handling of the various non-violent (diplomatic, scholastic and otherwise) interactions with Xenos individuals and powers, ranging from planet-wide populations down to individual Sanctioned Xenos - as well as humans that have not been inducted into the Imperium - from newly-rediscovered human colonies, to the illiterate feral worlder that does not know who the Emperor is. They issue the licenses to Humans allowing them to legally interact with Xenos (such as xenolinguists), conduct the sanctioning procedures to legitimize a Sanctioned Xenos as a servant of the Imperium, and handle most related issues such as establishing and managing embassies and arranging the (ideally) safe well-being of the Humans and Xenos involved - and do the same for human individuals, ambassadors, and powers that have not yet joined the Imperium.
Who are these people and why have I never heard of them?
The Ambassador Imperialis handle work that is both mundane and difficult to write as grimdark. This causes them to be quickly overlooked by many writers, game producers, etc. for pickings that are easier to make exciting, make grimdark, or write a story around in a 40k setting. Without exciting battles and glorious last stands, the tale of an oppressed factory worker trying and failing to provide for their family in modern 40k can still be full of grimdark, but the Ambassador Imperialis usually relate to neither battles nor the dystopian conditions which would be considered grimdark. This is because if the Ambassador Imperialis are succeeding at their job, no disruptions requiring outside intervention will occur and no battles will take place. If a battle is taking place, the Ambassador Imperialis officials will either not be present, be dead, or otherwise be irrelevant to most stories. Likewise, if a story involving Human-Xenos or Imperial-Human-Non-Imperial-Human interaction is grimdark, the Ambassador Imperialis and their officials often have to be absent by necessity of writing convenience.
In short, you don't find much material about the Ambassador Imperialis for the same reason you don't find a book where the protagonist Human and their secret Xenos lover fret about how their love is illegal and they are risking their lives meeting, but then remember they can go down to the immigration office and get a Resident Visa so they go to the office, the Xenos becomes Sanctioned, with the book ending in a few pages (and being mostly comprised of scenes of people filling out paperwork).
Examples of Ambassador Imperialis characters are:
- The person that fills out the sanctioning paperwork and issues the insignia (or markings) when a Rogue Trader employs a Sanctioned Xenos.
- The people that decided the Jokaero were non-hostile and filled out all the related paperwork to grant the species Sanctioned status.
- The less-purge-obsessed Ministorum preacher sent to convert rediscovered human colonies to the Imperial Cult (without burning the place down).
- The person that decided the above colony had valuable infrastructure the Imperium could use and either selected people that would not destroy said infrastructure, or gave them orders to avoid doing so.
- The guy that negotiated the peaceful integration of that Human-Xenos cohabitation world into the Imperium and Sanctioned the Xenos race, successfully and without things devolving into grimdark purges, civil war, or tremendous loss of life and valuable resources.
- The other countless people that did the same or similar to any of the above, in those stories that were not written, in those books that were not published.
The Ambassador Imperialis are around when things are going correctly, or are people actively trying to find non-destructive solutions to problems and preserve as many resources for the Imperium as possible - and in so doing, they may create an easy way for a prospective writer to trap themselves in a corner if such a character or organization is present in their story. As such, the characters listed above are characters that can be confirmed to exist, but go unnamed and unwritten about - despite the enormous amounts of power they may have - due to their job being to be a beacon of noblebright and a massive disruption to any grimdark story someone is trying to write. Think of them in a similar vein to how temples to Sanguinius are said to be as numerous and grand as temples to the Emperor, but no stories are made about a Cult of Sanguinius efficiently running a planet with healthcare and paid vacations for all.
This lack of writing interest combined with potential writing hazard has resulted in only the occasional indirect nod to their existence by virtue of their actions (such as in Rogue Trader's Into The Storm Sanctioning of Xenos) or their minor relevance to an overall story (such as in the Cain novels). The term Ambassador Imperialis itself comes from a relatively-small nod to their presence in the Dark Heresy Second Edition Enemies Without supplement, which lets a player take it as a Talent for improved interactions with non-imperials).
So what are they up to now?
As of the events of Gathering Storm, the logical inference would be that they are gaining more power and have probably benefited from the return of Roboute Guilliman due to a relatively-compatible outlook... but for now there are still few official materials on their activities, so we must hope a writer or two take an interest in them in the future.