13C. The name 'Boyar' shall obviously evoke the mysterious lands at the easternmost end of early modern Europe. At the turn of the 16th century, the historical Muscovy had fallen into a succession crisis known as the Time of Troubles because Feodor I, the last of the Rurik dynasty, died without leaving any male heir. The land was devastated by famine and by the civil war amongst the various claimants.
In 13C, a figure has emerged from this squabble to unite all the petty kingdoms east of the Oriental Range under his iron rule. Little is known about the Great Boyar, except that he commands ruthless hordes to do his bidding.
The Great Boyar is shrouded in mystery, and so are his whereabouts, although he is rumoured to dwell, attended by beautiful rusalki, at the centre of a vast and foreboding marsh guarded by fearsome vodyanyie.
The Great Boyar being a figure shrouded in mystery, and his lands being far and isolated, little is really known about him. Some say he's far taller than a normal person, some say he's not really human, some others that he is of an amphibious nature; some others yet insist all of this is true— and then worse. Yet some argue that all these rumours are purposely spread by the Great Boyar himself to instil a sense of fear in his enemies.
ADVENTURERS & THE ICON
At the moment, the influence of the Great Boyar doesn't really extend over the Fair Kingdom, so adventurers shouldn't feel his influence (yet). However, it is rumoured that the Great Boyar commands many aquatic creatures, such as the already-mentioned rusalki and vodyanyie, and he may have started to gain hold upon the waterways and the marshes of the north-eastern borderlands of the Fair Kingdom.
Also, adventurers who like to plunder more than law may look favourably upon the Great Boyar and the promises of looting that the onslaught of the Eastern hordes might bring upon the Fair Kingdom.
A particularly interesting PC background could be that of the exiled knyaz, a wandering knight pertaining to one of the noble houses on the losing side of the Muscovite civil war. These adventurers would have a conflicted or negative relationship with the Great Boyar.
The Great Boyar operates on his own. He's also a big unknown because he's a recent addition to the roster of Icons. The Emperor will certainly try and manoeuvre him so that he turns his attention towards the Empire of the Crescent Moon first, and the Sultan will try and do the same but with regards to the Empire of the One Faith.
No actual enemies yet. The Emperor is known to have tried to prevent the Great Boyar from reuniting the petty kingdoms of the East via his proxies there, but he has failed.
Little is known about what the Great Boyar was before gaining his current title.
THE TRUE DANGER
At the moment the Great Boyar is but a distant threat. However, the other Icons would do well to remember that the last time a single individual managed to unite the Eastern lands, almost all of Europe fell before the Eastern hordes. Will history repeat itself?
Should you want to run a mash-up of my Fair Kingdom setting and of the Land of Unreason from A Red & Pleasant Land, the Great Boyar could be the Twin of the Colourless Queen, or even her spouse, who manages to influence events in the Land of Unreason via its many canals.
In this case, the distant eastern domain ruled by the Great Boyar is the geographical twin of Nephilidia.