Let us take, for example, the research I've been doing for the story with dsgood's 5 words. According to one account, Samuil had administrative control of the First Bulgarian Empire from 976AD, and became Tsar in name as well. Another account holds that Samuil, in fact, was a Macedonian general who, as the Bulgarian empire collapsed, carved out of it's remnants, and out of a piece of the Byzantine empire, an independant Macedonian Empire. This theory holds that the Pope crowned Samuil "Tsar of Bulgaria" because the papacy was not in the position to "create" new countries, so would transfer to new countries names that had become essentially unused.
Why is this important?
Well, it has something to do with the fact that the city of Ohrid was a cultural and scholarly capital of the time (the cyrillic alphabet was developed there, etc). It has something to do with the cultural ethnicity of Alexander the Great. It has something to do with the linguistic origins of Macedonian names, and of the timing of slavic incursion into the region.
And why is this important?
Well, I think that while it's pretty clear that Alexander the Great wasn't slavic (assuming that the dates are right), he's very unlikely to have been Greek. Macedonian culture by the time of the turn of the millenium was likely a hybrid of slavic and indigenous cultures, with some influence of cultures of the east (Persia, Mongolia) filtering down from Bulgaria and Hungary. Present day Macedonia is split between three countries - the Republic of Macedonia (formerly part of Yugoslavia, and peopled with ethnic Macedonians and a significant Albanian muslim minority), Bulgaria and Greece. Greece in particular has been doing everything it can to eliminate the macedonianess of it's macedonians - the language was banned (in schools and in any governmental settings - i.e. it would be illegal to translate into or out of macedonian in a court of law), and nobody with a macedonian name was permitted to own property. Why? Alexander was Greek, dammit. WE had the greatest empire, not those stinking slavs. Also, there is a non-trivial possibility that Macedonians in Greece will make a bid to split off and join in a "Greater Macedonia."
And why is this important?
Because there's a significant minority of muslims in these areas. And if this were to happen, and the trend toward democracy in the region were to continue, then the muslims in Macedonia (north-western border) and Bulgaria (northern border) would have a voice, and would therefore impose upon those countries the will of the Turks (eastern border). And why does this matter? Greece is the descendant of the Byzantine Empire, which was "the real successor" of the Roman Empire, and which was the bastion of Christianity against the Persians and the Arabs and the Turks and the Muslims.
For the Macedonians, though, it is a matter of claiming what they consider their cultural heritage, and of solidifying their claim of independence from Serbia (Serbs and Greeks tend to view Macedonians as a sort of "lesser serb").
So... Anyway. Back to Samuil (also known as Samoil):
This second theory is held mostly by Macedonians and Serbs, which leads to a bit of suspicion. However, it does explain to some extent the dispersion pattern of the Bogomil Heresy (even though none of these histories really addresses the impact of said heresy). The Bogomil Heresy held that Jesus was not the only son of God. Jesus was an aspect of the second son of God, Michael. Michael was Satanail's younger brother. Satanail rebelled against God, was kicked out of His house, and then went on to create The World and It's Creatures. But he had trouble with humans, so God assisted Satan in creating humans. Since the earth belonged to Satan, humans made a covenant with Satan for permission to till the soil. Jesus was sent to earth to break that covenent and allow us to stop serving the evil one.
What influence did a bunch of heretics have? Well, part of their ideology was a rejection of the established clergy, of the orthodox church (created by Satan, it was), and of things of this world (created by Satan, they were). This meant that they refused to pay taxes, or work, or fight for their state, creating significant social disruption in Bulgaria and elsewhere.
Samoil's reign (whether it was Bulgarian or Macedonian) ended in 1014 in the battle of Belasitsa, when Basil II of Byzantium blinded 14,000 men, leaving 1 in 100 with 1 eye to lead them home.
The English word "bugger" is derived from the Bogomils.