Sepulchrave II
Legend
Heralds absolutely prefigured ambassadors; historically, they were generally given a good deal of room to negotiate - especially considering the speed of communication.It helps if you know the full meaning, rather than just read the first thing that pops up on an online dictionary. A herald, as used by Tolkien, is the representative of a ruler. It's almost the same as an ambassador, apart from a herald represents an individual, rather than a nation.
And an ambassador is a professional politician.
Another example, as used by Tolkien, is the Mouth of Sauron, who is Sauron's Herald.
All the stuff about heraldry evolved later, in the medieval period. But Tolkien was a Saxon scholar, not a medievalist.
Also, in the kind of feudal (or quasi-feudal) system to which Tolkien alludes, the political class (or ruling class) are the warrior class.
Interestingly, Eonwe, the herald of Manwe, did not have the authority to pardon Sauron at the end of the First Age - but by his own judgment. Sauron, presumably, thought he might - or he wouldn't have appealed to him.