Ok, where can I find info on numidian and the horn of summoning?
There's not much info at all on the Horn, the only text to mention it is http://www.imperial-library.info/content/daggerfall-real-barenziah but it implies it is quite powerful:
Before his death King Edward gave the horn for safekeeping into the hand of his old friend King Moraelyn, who secreted it here in Mournhold, under the guardianship of the god Ephen, whose birthplace this is...
[Jagar Tharn] knew the Staff [of Chaos] lay there, not the Horn, and that he must teleport to safety, for the Staff is not a weapon that would have seen him clear, unlike the horn. Praise the gods he hath not that! It seems all was as he expected, yet how did he know? I placed it there myself, with the aid of the rag-tail end of the R'Aathim clan who now sits king in Ebonheart as a reward. Tiber Septim claimed the Horn, but left the Staff for safe-keeping. Nightingale can use the Staff to sow seeds of strife and dissension, if he wishes, yet that alone will not gain him power. That lies with the Horn and the ability to use it."http://www.imperial-library.info/content/daggerfall-real-barenziah
There are three relevant points in this excerpt.
-The first is that it says the Horn is a weapon that "would have seen him clear," unlike the Staff. This could be taken to meant that the Horn could teleport him away somehow, however he can teleport of his own accord anyway, so this explanation is unlikely. Couple this with the fact that it implies it is a weapon, and we can assume that somehow the Horn can be used offensively - either directly or by, as the name implies, summoning 'something' to do it for him.
-The second thing to note is the exclamation made by Symmachus, "Praise the gods he hath not that!" We know from the events of Arena that the Staff of Chaos is a pretty handy little trinket, and while they're distraught that he got the staff, they're much more relieved that he didn't have the chance to get the horn. Again implying that the horn is much more powerful.
-The third note is the last line, which points out that the ability to gain power "lies with the Horn and the ability to use it."
-A fourth minor note is that the text has Tiber Septim claiming the Horn for himself, which might imply that it had some significant influence over Septim's rise to power.
That's really all the info there is on it, and its present location is unknown.