Mouth of Sauron at Erebor ?

[Legate of Amon Lanc]

I'd agree with Kuruharan&Boro on that this "horseman in the night" might be a Ringwraith. Boro's "hissing" points are quite good, and also Kuru's thoughts of Dol Guldur being much closer to Erebor than Mordor.

I went a little bit further and done a little exegesis to support this theory.

As we know, Gloins speaks of this messenger to Dain at the Elrond's Council; Gloin says that the messenger will come

for the third and the last time, as he said, before the end of the year.

This means that it was three times the same one, just to make matters clear (unless the Dwarves could not distinct one messenger from another, which is not impossible, a guy in black and a guy in black look the same, but on the other hand you don't send three different negotiators to Erebor if you have one who already was there and this is his job).

In Unfinished Tales, there is written, that

the second greatest [ringwraith], Khamul, the Shadow of the East, stayed in Dol Guldur along with his messenger

Well, I'm sorry for the unexact quotations, but if the second guy (a Ringwraith as well) is called "a messenger" and stays in Dol Guldur, this would speak for Boro's idea and imply that this pal of Khamul might very possibly be the one we seek.

If someone needed to verify if the Ringwraith were able to be at Erebor, there are the datums. Remember that it was almost at the start of their quest for the Ring-bearer, so were they "on-line" at the time the messenger came to Erebor?
Gloin says that first time the messenger came "a year before" the Council, as he puts it, so let's say september to november 3017. This would imply that immediately after receiving the information from Gollum, Sauron sent this messenger to the "scene of murder", Erebor (of course he did, being it such a grave matter as the One Ring). At this time, the Nazgul were certainly "free of duties" to do this. The second coming, however, is quite obscure. Gloin says just that he returned without answer, but does not say when. If our "horseman in the night" indeed was a Ringwraith, it would have to happen before June 3018, when the Witch-King and the other Ringwraith set out from Mordor to meet with Khamul and his messenger (according to UT). This happened after Sauron received the message of Gollum being captured by Aragorn, which, according to UT, was some time around April 3018. According to Gloin, the messenger said that he'd come before the end of the year. Sauron's choice of sending the Nazgul out was improvised and quick in response to the events at that time, thus, when the messenger came, he still counted on that the third time (as he said, before the end of 3018), he will return and ask about the whereabouts of the Ring and it's bearer. He did not know that at the moment, the whereabouts of the Ring would be already known because of speeding up the events with Sauron's response to the capture of Gollum by Aragorn.
So it is indeed possible that this messenger was a Ringwraith, and we also came up to the dates of the messenger's coming

So, to be precise, the only thing which might stand against the theory of the messenger being a Ringwraith would be that the Ringwraith were something like a "last resort" for Sauron, and he didn't want to use them because the fear they produced was too great. However, this fear was just the thing Sauron used (um, as always). Also, it was problem for the Ringwraith to cross water, however, this would be possible on oars or whatever in the Wilderness. They had also problem with orientation in daylight - but this would speak FOR the Ringwraith theory, since the messenger could have traveled at night - and, Gloin says, that he came in the night - and what man would not camp outside Erebor and wait for the dawn until negotiating! Also, in the night, his identity could not have been revealed - so that no one would know he was a Ringwraith!