The Prince of Pereyaslavl was the ruler of the Principality of Pereyaslavl, a lordship based on the city of Pereyaslavl on the Trubizh River, and straddling extensive territory to the east in what are now parts of Ukraine. It was situated on the southern frontier of Kievan Rus' and bordered the steppe. 
History.
The origins of the principality and the city of Pereyaslavl' itself are uncertain, with the "Primary Chronicle" mentioning "Pereyaslavl'" already just before the Rus'–Byzantine Treaty (907) as a prosperous town within Kievan Rus'. Yet, "sub anno" 993, the "Chronicle" claims that Vladimir the Great founded a town called "Pereyaslavl'" that year on the banks of the river Trubizh, a tributary of the Dnieper, south of Kiev and Chernigov. Either Pereyaslavl' was founded already, or Vladimir built a new city by the same name. 
After the Battle of Listven (1024), Vladimir's sons divided the realm along the Dnieper: Yaroslav the Wise reigned the right (west) bank including Novgorod from Kiev, while the left (east) bank including Pereyaslavl' were the domain Mstislav of Chernigov. The status of Pereyaslavl' remained unclear until 1054, when Yaroslav identified it as a principality within Kievan Rus', and appointed his son Vsevolod Yaroslavich as its prince.
His brother Svyatoslav received Chernigov, while Smolensk went to Vyacheslav and Vladimir-in-Volhynia to Igor. This ladder of succession is related to the seniority order mentioned above. Vsevolod's appanage included the northern lands of Rostov and the lightly colonised northeast (see Vladimir-Suzdal).
The "Primary Chronicle" recorded that in 988, Vladimir had assigned the northern lands (later associated with Pereyaslavl) to Yaroslav. 
The town was destroyed by the Mongols in March 1239, the first of the great Rus cities to fall. Certainly from the reign of Vsevolod Yaroslavich, the princes of Pereyaslavl held the principality of Rostov-Suzdal, which was heavily colonized by Slavs thereafter, a process which strengthened the region's power and independence, separating the two regions.
In 1132, Yaropolk became Grand Prince on his brother Mstislav's death, while the Monomashichi descended into general internecine conflict over the Pereyaslavl principality. Yaropolk appointed Vsevolod Mstislavich, prince of Novgorod, to the principality of Pereyaslavl – in this era designated heir to the Kievan throne – thus provoking Yaropolk's younger brother Yuri Dolgoruki, controller of Suzdal, into war. Yuri drove out Vsevolod, whom Yaropolk then replaced with Izyaslav. An agreement was reached by 1134 between Yuri and Yaropolk that their common brother Vyacheslav would take the throne of Pereyaslavl.
References.
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