Let us grant an audience to His Gracious Excellency Lord Dius Retashaza, Ambassador Extraordinary of the Court of Poseidon.
Let us, as true postmoderns, start by deconstructing his name.
He is an excellency because he is an ambassador. Ambassadors are always excellencies. Different ambassadors excel at different things. Dius Retashaza excels at the use of words. Perfectly ordinary words seem to take on new, extraordinary meanings when Dius Retashaza uses them.
He is gracious because he is the younger brother of a duke. Specifically, he is the younger brother of Duke Retashaza, one of the nine dukes of Atlantis. You can't get much closer to the Emperor than that without getting into bed with him. Rumour has it that it is Dius Retashaza's niece who does that.
Retashaza is an ambassador of the Court of Poseidon because that is how the Imperial Court of Atlantis styles itself. Prince Atlantis Atlantor 22nd sits on the Throne of Poseidon and is the de jure, if not the de facto, High Priest of Poseidon in Atlantis.
Hang on, you may say. If you are interested in politics, you may have been led to believe that it is the wizards of the Institutes of Atlantis who now rule that city. The wizards themselves certainly believe it. So why is it not they for whom Dius Retashaza speaks? The answer is simply that wizards have no experience of appointing ambassadors. They do not know how to go about it. Whereas the Court of Poseidon has been appointing ambassadors for centuries. So the wizards, who are not really all that interested in what happens outside Atlantis, let the Court of Poseidon get on with what it does best: foreign policy.
As it happens, the wizards are particularly happy with Dius Retashaza. It was he who sponsored the publication of the first ever Dictionary of the Atlantean Tongue (published by Wswfirn and Sons of Atlantis, price 5 crowns, available in all good bookshops). Anyone who gives money to scholars must have his heart in the right place.
Finally, Dius Retashaza is Extraordinary because he has not been appointed to a particular court. He is a floating ambassador, representing Atlantis to the entire universe.
[AD&D-Specific] Dius Retashaza, human male, Str 13 Dex 12 Con 14 Int 16 Wis 11 Cha 17 Align LE Hp 20. Fighter, level 2.
Dius Retashaza's alignment is magically-concealed - the only way of determining it is by observing his behaviour. Note, in particular, that he will not register on detect evil.
Retashaza knows just about enough swordsmanship to know one end of a sword from another. For the uninitiated, the pointy end is the one you stick into the other guy. When the customs of his host land permit it, he will wear a very expensive-looking katana which registers very strongly on detect magic. It has been in the Retashaza family for several generations, and it is actually +1, +4 vs reptiles.
In principle, Retashaza could advance in level as a fighter. It will not happen quickly, as Retashaza will avoid combat wherever possible. Even against reptiles.
Retashaza's principal weapon is words. Although he always keeps his promises, they often turn out not to mean what you thought they meant.
He doesn't look very scary, does he?
He is, though.
Firstly, Retashaza will never travel alone. With him will be 5-10 warriors of approximately 7th level and a wizard of 8th or 9th level. The warriors are hirelings. The wizard is one of two who used to be the family wizard serving the Retashaza family before retiring (at 9th level) or moving to private spell research (at 8th level).
Secondly, Retashaza can call upon the services of the Atlantean Navy. If he is anywhere near a coastline, there is likely to be at least one Atlantean naval vessel within summoning distance. Treat this as a buccaneer ship (see the entry for human in the Monster Manual). The captain might resent being summoned in this way, or he might be looking forward to seeing some real action at last. Either way, he and his officers will obey Retashaza's instructions.
Thirdly, Retashaza has some interesting friends. One such is Talaialam Alie, an adventuring diviner from the Institute of Words. She has an unrequited lust for him, and would do almost anything he asked of her.
Fourthly, any threat to Retashaza is a threat to Atlantis. There may be times when he fails to achieve his objectives; that is part and parcel of the game of diplomacy. But if anyone does harm to Retashaza's person, that is not part and parcel of the game of diplomacy, that is war. If Atlantis ever went to war, recalling all the Atlantean-born mercenaries serving in armies everywhere in the Fifty Worlds to form the core of its army, it could wipe out most city states very quickly.
Retashaza is, essentially, a salesman. What he is selling is the ideal of Atlantis as a peaceful and honest trading partner.
Retashaza could appear anywhere in the Fifty Worlds, but he is more likely to be on the world of Delta than anywhere else, and he is more likely to be close to a coastline than in the desert or mountains. Given the number of spies in Omnatia, he is particularly likely to turn up there.
If the players wish to have a political campaign, Retashaza could become an important NPC. If they don't, he still has to be there in the background, because it is unthinkable that Atlantis would not have a roving ambassador.
If you want to start a world war very quickly, have someone bump him off.