Slevsky has expanded over the last few centuries, particularly after the Day of Blood, when the royals were assassinated save for the young Garyth Creed. The Baron in those days was replaced by an undead creature who took on his appearance, and made a grab for power during Temer's attempted summoning of Cthulu. The northern part of this Barony was briefly endangered by a rampaging Tarrasque during this period, as well, before it returned to some lair deep under the mountains to settle. The barony has, for sure, seen its share of world-changing events.
Over the next two and a half centuries, the barony annexed the remaining two in the southeast of Celdun, wearing them down economically and through other dubious means until they agreed to assimilation.
Now, the barony has forcibly pushed the impression of them being the villains of Celdun into the past, and are working on a bright future. The Tinker's Guild, still Aithar's predominant technology-focused organization, has its headquarters in the Slevsky Barony. The Tinkers, however, follow a much more refined and restrained code since the monarchy has been back in control, and are no longer capable of enforcing a monopoly on technological advancement in the country.
The capital city of Celdun, Harreton, is here. There has been talk here and there of shifting the capital to the Iron City in the north, but no serious plans have ever been made public on the matter.
Baron Dariel Slevsky currently rules here. The Slevsky barons have a long history of adopting the Slevsky name after they seize power. This barony has been in familial dynasties many times throughout its history, often for several generations before a power shift.